Semiconductor device

ABSTRACT

When a semiconductor device including a transistor in which a gate electrode layer, a gate insulating film, and an oxide semiconductor film are stacked and a source and drain electrode layers are provided in contact with the oxide semiconductor film is manufactured, after the formation of the gate electrode layer or the source and drain electrode layers by an etching step, a step of removing a residue remaining by the etching step and existing on a surface of the gate electrode layer or a surface of the oxide semiconductor film and in the vicinity of the surface is performed. The surface density of the residue on the surface of the oxide semiconductor film or the gate electrode layer can be 1×10 13  atoms/cm 2  or lower.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film (also referred to as a thin film transistor(TFT)) formed over a substrate having an insulating surface. Thetransistor is applied to a wide range of electronic devices such as anintegrated circuit (IC) and an image display device (display device). Asilicon-based semiconductor material is widely known as a material ofthe semiconductor thin film applicable to the transistor. As anothermaterial, an oxide semiconductor has been attracting attention.

For example, a transistor including a semiconductor layer formed of anamorphous oxide including indium (In), gallium (Ga), and zinc (Zn) (anIn—Ga—Zn—O-based amorphous oxide) is disclosed (see Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-181801

SUMMARY OF THE INVENTION

Improvement in reliability is important for commercialization ofsemiconductor devices including transistors that include an oxidesemiconductor.

However, a semiconductor device includes a plurality of thin filmscomplicatedly stacked, and is manufactured using a variety of materials,methods, and steps. Therefore, an employed manufacturing process maycause shape defects or a degradation of electrical characteristics ofthe semiconductor device.

In view of the above problem, it is an object to provide a highlyreliable semiconductor device including a transistor using an oxidesemiconductor.

It is another object to manufacture a highly reliable semiconductordevice with high yield to improve productivity.

In a semiconductor device including an inverted staggered bottom-gatetransistor according to an embodiment of the present invention, asurface of an oxide semiconductor film or a surface of a gate electrodelayer and the vicinity thereof are prevented from being contaminated bya residue that remains by an etching step for forming a metal layer (thegate electrode layer or a source and drain electrode layers).

In the etching step for forming the metal layer such as the gateelectrode layer or the source and drain electrode layers, a residue ofan etchant (an etching gas or an etching solution) remains on a surfaceof the metal layer or a surface of the oxide semiconductor film and inthe vicinity thereof. This residue causes a reduction or variation inelectrical characteristics of the transistor, such as a decrease inwithstand voltage of a gate insulating film or an increase in leakagecurrent.

The residue includes the etchant (the etching gas or the etchingsolution), the processed metal layer, an element contained in the oxidesemiconductor film which is exposed to the etchant, and a compound ofsuch an element. For example, a gas including halogen is favorably usedin the etching step for forming the metal layer such as the gateelectrode layer or the source and drain electrode layers; in that case,the residue is a halogen impurity (halogen or a halide).

As the residue, chlorine, fluorine, boron, phosphorus, aluminum, iron,carbon, or the like can be given, for example. In addition, the residuemay include a metal element (e.g., indium, gallium, or zinc) included inthe oxide semiconductor film or the like, in some cases.

In an embodiment of a structure of the invention disclosed in thisspecification, after the source electrode layer and the drain electrodelayer are formed, a step of removing the residue existing on the surfaceof the oxide semiconductor film and in the vicinity of the surfacebetween the source electrode layer and the drain electrode layer (aresidue removal step) is performed.

In another embodiment of the structure of the invention disclosed inthis specification, after the gate electrode layer is formed, a step ofremoving the residue existing on the surface of the gate electrode layer(a residue removal step) is performed.

Treatment using water or an alkaline solution or plasma treatment can beperformed as the residue removal step. Specifically, treatment usingwater or a tetramethylammonium hydroxide (TMAH) solution or plasmatreatment using oxygen, dinitrogen monoxide, or a rare gas (typicallyargon) can be favorably used. Alternatively, treatment using dilutehydrofluoric acid may be used.

Since the surface of the oxide semiconductor film or the surface of thegate electrode layer and the vicinity of the surface can be preventedfrom being contaminated by the residue, in the semiconductor deviceincluding the inverted staggered bottom-gate transistor, the surfacedensity of the residue (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of theoxide semiconductor film (or the gate electrode layer) can be 1×10¹³atoms/cm² or lower (preferably 1×10¹² atoms/cm² or lower). Further, theconcentration of the residue (typically halogen (e.g., chlorine,fluorine), boron, phosphorus, aluminum, iron, or carbon) on the surfaceof the oxide semiconductor film (or the gate electrode layer) can be5×10¹⁸ atoms/cm³ or lower (preferably 1×10¹⁸ atoms/cm³ or lower).

Accordingly, a highly reliable semiconductor device including atransistor using an oxide semiconductor film and having stableelectrical characteristics can be provided. In addition, the highlyreliable semiconductor device can be manufactured with high yield, sothat productivity can be improved.

An embodiment of the structure of the invention disclosed in thisspecification is a semiconductor device which includes a gate electrodelayer over an insulating surface, a gate insulating film over the gateelectrode layer, an oxide semiconductor film over the gate insulatingfilm, a source electrode layer and a drain electrode layer over theoxide semiconductor film, and an insulating film which is in contactwith a region of the oxide semiconductor film overlapping with the gateelectrode layer and covers the source electrode layer and the drainelectrode layer. In the semiconductor device, a surface of the oxidesemiconductor film is in contact with the insulating film. The surfacehas a surface density of halogen of 1×10¹³ atoms/cm² or lower.

Another embodiment of the structure of the invention disclosed in thisspecification is a semiconductor device which includes a gate electrodelayer over an insulating surface, a gate insulating film over the gateelectrode layer, an oxide semiconductor film over the gate insulatingfilm, a source electrode layer and a drain electrode layer over theoxide semiconductor film, and an insulating film which is in contactwith a region of the oxide semiconductor film overlapping with the gateelectrode layer and covers the source electrode layer and the drainelectrode layer. In the semiconductor device, a surface of the gateelectrode layer has a surface density of halogen of 1×10¹³ atoms/cm² orlower.

Another embodiment of the structure of the invention disclosed in thisspecification is a semiconductor device which includes a gate electrodelayer over an insulating surface, a gate insulating film over the gateelectrode layer, an oxide semiconductor film over the gate insulatingfilm, a source electrode layer and a drain electrode layer over theoxide semiconductor film, and an insulating film which is in contactwith a region of the oxide semiconductor film overlapping with the gateelectrode layer and covers the source electrode layer and the drainelectrode layer. In the semiconductor device, a surface of the oxidesemiconductor film is in contact with the insulating film. The surfaceof the oxide semiconductor film has a surface density of halogen of1×10¹³ atoms/cm² or lower, and a surface of the gate electrode layer hasa surface density of halogen of 1×10¹³ atoms/cm² or lower.

Another embodiment of the structure of the invention disclosed in thisspecification is a method for manufacturing a semiconductor device whichincludes the steps of forming a gate electrode layer over an insulatingsurface, forming a gate insulating film over the gate electrode layer,forming an oxide semiconductor film over the gate insulating film,forming a conductive film over the oxide semiconductor film, forming asource electrode layer and a drain electrode layer by etching theconductive film using a gas including halogen, and performing a residueremoval step on the oxide semiconductor film.

Another embodiment of the structure of the invention disclosed in thisspecification is a method for manufacturing a semiconductor device whichincludes the steps of forming a conductive film over an insulatingsurface, forming a gate electrode layer by etching the conductive filmusing a gas including halogen; performing a residue removal step on thegate electrode layer; forming a gate insulating film over the gateelectrode layer after the step of performing the residue removal step onthe gate electrode layer, forming an oxide semiconductor film over thegate insulating film, and forming a source electrode layer and a drainelectrode layer over the oxide semiconductor film.

Another embodiment of the structure of the invention disclosed in thisspecification is a method for manufacturing a semiconductor device whichincludes the steps of forming a first conductive film over an insulatingsurface, forming a gate electrode layer by etching the first conductivefilm using a gas including halogen, performing a residue removal step onthe gate electrode layer, forming a gate insulating film over the gateelectrode layer after the step of performing the residue removal step onthe gate electrode layer, forming an oxide semiconductor film over thegate insulating film, forming a second conductive film over the oxidesemiconductor film, forming a source electrode layer and a drainelectrode layer by etching the second conductive film using a gasincluding halogen, and performing a residue removal step on the oxidesemiconductor film.

An embodiment of the present invention relates to a semiconductor deviceincluding a transistor or a semiconductor device including a circuitwhich is formed by using a transistor. For example, an embodiment of thepresent invention relates to a semiconductor device including atransistor in which a channel formation region is formed using an oxidesemiconductor or a semiconductor device including a circuit which isformed by using such a transistor. For example, an embodiment of thepresent invention relates to an electronic device which includes, as acomponent, an LSI; a CPU; a power device mounted in a power circuit; asemiconductor integrated circuit including a memory, a thyristor, aconverter, an image sensor, or the like; an electro-optical devicetypified by a liquid crystal display panel; or a light-emitting displaydevice including a light-emitting element.

With an embodiment of the present invention, a highly reliablesemiconductor device including a transistor using an oxide semiconductoris provided.

With an embodiment of the present invention, a highly reliablesemiconductor device is manufactured with high yield, so thatproductivity is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E are cross-sectional views illustrating an embodiment of asemiconductor device and a method for manufacturing the semiconductordevice;

FIGS. 2A to 2E are cross-sectional views illustrating an embodiment of asemiconductor device and a method for manufacturing the semiconductordevice;

FIGS. 3A to 3F are cross-sectional views illustrating an embodiment of asemiconductor device and a method for manufacturing the semiconductordevice;

FIGS. 4A to 4C are plan views illustrating examples of a semiconductordevice;

FIGS. 5A and 5B are a plan view and a cross-sectional view,respectively, illustrating an example of a semiconductor device;

FIGS. 6A and 6B are cross-sectional views illustrating examples of asemiconductor device;

FIGS. 7A and 7B are a circuit diagram and a cross-sectional view,respectively, illustrating an embodiment of a semiconductor device;

FIGS. 8A to 8C illustrate electronic devices; and

FIGS. 9A to 9C illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed in this specification will bedescribed below with reference to the accompanying drawings. Note thatthe invention disclosed in this specification is not limited to thefollowing description, and it is easily understood by those skilled inthe art that modes and details can be variously changed withoutdeparting from the spirit and the scope of the invention. Therefore, theinvention disclosed in this specification is not construed as beinglimited to the description of the following embodiments. Note that theordinal numbers such as “first” and “second” in this specification areused for convenience and do not denote the order of steps and thestacking order of layers. In addition, the ordinal numbers in thisspecification do not denote particular names which specify the presentinvention.

Embodiment 1

In this embodiment, an embodiment of a semiconductor device and a methodfor manufacturing the semiconductor device will be described withreference to FIGS. 1A to 1E. In this embodiment, a semiconductor deviceincluding a transistor including an oxide semiconductor film will bedescribed as an example of the semiconductor device.

The transistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned above and below a channel formation region with a gateinsulating film provided therebetween.

A transistor 440 illustrated in FIG. 1E is an example of an invertedstaggered transistor that is one type of a bottom-gate transistor. Notethat FIGS. 1A to 1E are cross-sectional views taken along a channellength direction of the transistor 440.

As illustrated in FIG. 1E, the semiconductor device including thetransistor 440 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating film 402, anoxide semiconductor film 403, a source electrode layer 405 a, and adrain electrode layer 405 b. An insulating film 407 is provided to coverthe transistor 440.

An oxide semiconductor used for the oxide semiconductor film 403contains at least indium (In). In particular, In and zinc (Zn) arepreferably contained. In addition, as a stabilizer for reducingvariation in electrical characteristics of a transistor formed using theoxide semiconductor film, gallium (Ga) is preferably contained inaddition to In and Zn. Tin (Sn) is preferably contained as a stabilizer.Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) ispreferably contained as a stabilizer. Zirconium (Zr) is preferablycontained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, tin oxide, zinc oxide, an In—Zn-based oxide, anIn—Mg-based oxide, an In—Ga-based oxide, an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and anIn—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than the In, Ga, and Zn.

A material represented by InMO₃(ZnO)_(m) (m>0, where m is not aninteger) may be used as the oxide semiconductor. Note that M representsone or more metal elements selected from Ga, Fe, Mn, and Co.Alternatively, a material represented by In₂SnO₅(ZnO)_(n) (n>0, where nis an integer) may be used as the oxide semiconductor.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), orIn:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or an oxide with an atomic ratio in theneighborhood of the above atomic ratios can be used. Alternatively, anIn—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1(=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5(=1/4:1/8:5/8), or an oxide with an atomic ratio in the neighborhood ofthe above atomic ratios may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used as the oxide semiconductorcontaining indium depending on needed electrical characteristics (e.g.,mobility, threshold voltage, and variation). In order to obtain theneeded electrical characteristics, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio between a metal element and oxygen, the interatomic distance, thedensity, and the like be set to appropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn-based oxide. However, mobility can also beincreased by reducing the defect density in a bulk in the case of usingan In—Ga—Zn-based oxide.

For example, in the case where the composition of an oxide containingIn, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in theneighborhood of the composition of an oxide containing In, Ga, and Zn atthe atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1), a, b, and c satisfy thefollowing relation: (a−A)²+(b−B)²+(c−C)²≤r², and r may be 0.05, forexample. The same applies to other oxides.

The oxide semiconductor film 403 is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film 403 is preferably a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts andamorphous parts are included in an amorphous phase. Note that in mostcases, the crystal part fits inside a cube whose one side is less than100 nm. From an observation image obtained with a transmission electronmicroscope (TEM), a boundary between an amorphous part and a crystalpart in the CAAC-OS film is not necessarily clear. Further, with theTEM, a grain boundary is not observed in the CAAC-OS film. Thus, in theCAAC-OS film, a reduction in electron mobility, due to the grainboundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°. Notethat part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in the impurity-added region becomes amorphous in somecases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

With use of the CAAC-OS film in a transistor, change in electricalcharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Thus, the transistor has highreliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

In an oxide semiconductor having a crystal part such as the CAAC-OSfilm, defects in the bulk can be further reduced and when the surfaceflatness of the oxide semiconductor is improved, mobility higher thanthat of an oxide semiconductor in an amorphous state can be obtained. Inorder to improve the surface flatness, the oxide semiconductor ispreferably formed over a flat surface. Specifically, the oxidesemiconductor may be formed over a surface with an average surfaceroughness (R_(a)) of less than or equal to 1 nm, preferably less than orequal to 0.3 nm, further preferably less than or equal to 0.1 nm.

Note that R_(a) is obtained by expanding arithmetic mean surfaceroughness, which is defined by JIS B 0601: 2001 (ISO4287: 1997), intothree dimensions so as to be applied to a curved surface. R_(a) can beexpressed as an “average value of the absolute values of deviations froma reference surface to a designated surface” and is defined by thefollowing formula.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}{dxdy}}}}}} & \left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂,f(x₁, y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). S₀ representsthe area of a rectangle which is obtained by projecting the designatedsurface on the xy plane, and Z₀ represents the height of the referencesurface (the average height of the designated surface). R_(a) can bemeasured using an atomic force microscope (AFM).

In addition, the reference surface is a surface parallel to the xy planeat the average height of the designated surface. In short, when theaverage value of the height of the designated surface is denoted by Z₀,the height of the reference surface is also represented by Z₀.

Note that since the transistor 440 described in this embodiment is abottom-gate transistor, the substrate 400, the gate electrode layer 401,and the gate insulating film 402 are located below the oxidesemiconductor film. Accordingly, planarization treatment such as CMP(chemical mechanical polishing) treatment may be performed after theformation of the gate electrode layer 401 and the gate insulating film402 to obtain the above flat surface.

The oxide semiconductor film 403 has a thickness greater than or equalto 1 nm and less than or equal to 30 nm (preferably greater than orequal to 5 nm and less than or equal to 10 nm) and can be formed by asputtering method, a molecular beam epitaxy (MBE) method, a CVD method,a pulse laser deposition method, an atomic layer deposition (ALD)method, or the like as appropriate. The oxide semiconductor film 403 maybe formed with a sputtering apparatus which performs deposition in thestate where surfaces of a plurality of substrates are substantiallyperpendicular to a surface of a sputtering target.

An example of a method for manufacturing the semiconductor deviceincluding the transistor 440 is illustrated in FIGS. 1A to 1E.

There is no particular limitation on the substrate that can be used asthe substrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

A flexible substrate may be used as the substrate 400 to manufacture thesemiconductor device. To manufacture a flexible semiconductor device,the transistor 440 including the oxide semiconductor film 403 may bedirectly formed over a flexible substrate; or alternatively, thetransistor 440 including the oxide semiconductor film 403 may be formedover a substrate and then may be separated and transferred to a flexiblesubstrate. Note that in order to separate the transistor 440 from themanufacturing substrate and transfer it to the flexible substrate, aseparation layer may be provided between the manufacturing substrate andthe transistor 440 including the oxide semiconductor film.

As a base film, an insulating film may be provided over the substrate400. The insulating film can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film of siliconoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafniumoxide, or gallium oxide; a nitride insulating film of silicon nitride,silicon nitride oxide, aluminum nitride, or aluminum nitride oxide; or afilm of a mixed material of any of the above materials.

Heat treatment may be conducted on the substrate 400 (or both thesubstrate 400 and the insulating film). For example, the heat treatmentmay be conducted with a GRTA (gas rapid thermal annealing) apparatusthat performs heat treatment using high-temperature gas, at 650° C. for1 minute to 5 minutes. As the high-temperature gas used in GRTA, aninert gas which does not react with a processing object by the heattreatment, such as nitrogen or a rare gas like argon, is used.Alternatively, the heat treatment may be conducted with an electricfurnace at 500° C. for 30 minutes to 1 hour.

Next, a conductive film is formed over the substrate 400 and is etched,so that the gate electrode layer 401 is formed. Note that the etching ofthe conductive film may be performed by dry etching, wet etching, orboth of them.

The gate electrode layer 401 can be formed using a metal material suchas molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component. Alternatively, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the gate electrode layer 401. The gate electrodelayer 401 may have a single-layer structure or a stacked-layerstructure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide, indium tin oxide includingtitanium oxide, indium tin oxide to which silicon oxide is added, indiumzinc oxide, or indium zinc oxide including tungsten oxide. It is alsopossible that the gate electrode layer 401 has a stacked-layer structureof the above conductive material and the above metal material.

As the gate electrode layer 401 which is in contact with the gateinsulating film 402, a metal oxide including nitrogen, specifically, anIn—Ga—Zn-based oxide film including nitrogen, an In—Sn-based oxide filmincluding nitrogen, an In—Ga-based oxide film including nitrogen, anIn—Zn-based oxide film including nitrogen, a tin oxide film includingnitrogen, an indium oxide film including nitrogen, or a metal nitride(InN, SnN, or the like) film can be used. These films each have a workfunction of 5 eV (electron volts) or higher, preferably 5.5 eV orhigher, which enables the threshold voltage, which is one of electriccharacteristics of a transistor, to be positive when used as the gateelectrode layer.

In this embodiment, a tungsten film with a thickness of 100 nm is formedby a sputtering method.

Heat treatment may be conducted on the substrate 400 and the gateelectrode layer 401 after the formation of the gate electrode layer 401.For example, the heat treatment may be conducted with a GRTA apparatusat 650° C. for 1 minute to 5 minutes. Alternatively, the heat treatmentmay be conducted with an electric furnace at 500° C. for 30 minutes to 1hour.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401.

To improve the coverage by the gate insulating film 402, planarizationtreatment may be performed on a surface of the gate electrode layer 401.In the case of using a thin insulating film as the gate insulating film402 in particular, it is preferable that the flatness of the surface ofthe gate electrode layer 401 be good.

The gate insulating film 402 can be formed to have a thickness ofgreater than or equal to 1 nm and less than or equal to 20 nm by asputtering method, an MBE method, a CVD method, a pulse laser depositionmethod, an ALD method, or the like as appropriate. The gate insulatingfilm 402 may be formed with a sputtering apparatus that performs filmformation with surfaces of a plurality of substrates set substantiallyperpendicular to a surface of a sputtering target.

A material of the gate insulating film 402 can be a silicon oxide film,a gallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film.

When the gate insulating film 402 is formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen isadded, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,gate leakage current can be reduced. Further, the gate insulating film402 may have either a single-layer structure or a stacked-layerstructure.

It is preferable that the gate insulating film 402 include oxygen in aportion that is in contact with the oxide semiconductor film 403. Inparticular, the gate insulating film 402 preferably includes, in thefilm (bulk), an amount of oxygen which exceeds at least the amount ofoxygen in the stoichiometric composition. For example, in the case wherea silicon oxide film is used as the gate insulating film 402, thecomposition formula of the gate insulating film 402 is SiO_(2+a) (α>0).

When the gate insulating film 402 including much (an excessive amountof) oxygen, which serves as an oxygen supply source, is provided incontact with the oxide semiconductor film 403, oxygen can be suppliedfrom the gate insulating film 402 to the oxide semiconductor film 403.Heat treatment may be performed in the state where the oxidesemiconductor film 403 is at least partly in contact with the gateinsulating film 402, to supply oxygen to the oxide semiconductor film403.

By supply of oxygen to the oxide semiconductor film 403, oxygenvacancies in the film can be filled. Further, the gate insulating film402 is preferably formed in consideration of the size of the transistorand the step coverage by the gate insulating film 402.

In this embodiment, a silicon oxynitride film with a thickness of 200 nmis formed by a high-density plasma CVD method.

After the formation of the gate insulating film 402, heat treatment maybe conducted on the substrate 400, the gate electrode layer 401, and thegate insulating film 402. For example, the heat treatment may beconducted with a GRTA apparatus at 650° C. for 1 minute to 5 minutes.Alternatively, the heat treatment may be conducted with an electricfurnace at 500° C. for 30 minutes to 1 hour.

Next, the oxide semiconductor film 403 is formed over the gateinsulating film 402 (see FIG. 1A).

In order that hydrogen or water will not enter the oxide semiconductorfilm 403 as much as possible in the formation step of the oxidesemiconductor film 403, it is preferable to preheat the substrateprovided with the gate insulating film 402 in a preheating chamber of asputtering apparatus as a pretreatment for formation of the oxidesemiconductor film 403, so that impurities such as hydrogen and moistureadsorbed onto the substrate and the gate insulating film 402 areeliminated and exhausted. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferable.

Planarization treatment may be performed on the region of the gateinsulating film 402 which is to be in contact with the oxidesemiconductor film 403. There is no particular limitation on theplanarization treatment, and polishing treatment (e.g., CMP treatment),dry etching treatment, or plasma treatment can be used.

As the plasma treatment, reverse sputtering in which an argon gas isintroduced and plasma is generated can be performed. The reversesputtering is a method in which voltage is applied to a substrate sidewith use of an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a surface ismodified. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, an oxygen atmosphere, or the like maybe used. The reverse sputtering can remove particle substances (alsoreferred to as particles or dust) attached to the surface of the gateinsulating film 402.

As the planarization treatment, polishing treatment, dry etchingtreatment, or plasma treatment may be performed a plurality of times, orthese treatments may be performed in combination. In the case where thetreatments are combined, the order of steps is not particularly limitedand may be set as appropriate depending on the roughness of the surfaceof the gate insulating film 402.

The oxide semiconductor film 403 is preferably formed under conditionssuch that much oxygen is included during deposition (for example, by asputtering method in an atmosphere where the proportion of oxygen is100%) so as to be a film including much oxygen (preferably including aregion that includes an excessive amount of oxygen that exceeds theamount of oxygen in the stoichiometric composition of the oxidesemiconductor in a crystalline state).

In this embodiment, as the oxide semiconductor film 403, anIn—Ga—Zn-based oxide film (an IGZO film) is formed with a thickness of35 nm by a sputtering method using a sputtering apparatus that includesan AC power supply device. In this embodiment, an In—Ga—Zn-based oxidetarget with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) is used.The deposition conditions are as follows: the atmosphere is oxygen andargon (the flow rate of oxygen: 50%), the pressure is 0.6 Pa, the powersupply is 5 kW, and the substrate temperature is 170° C. The depositionrate under these deposition conditions is 16 nm/min.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as thesputtering gas for the deposition of the oxide semiconductor film 403.

The substrate is held in a deposition chamber kept under reducedpressure. Then, a sputtering gas from which hydrogen and moisture areremoved is introduced into the deposition chamber while remainingmoisture is removed from the deposition chamber, and the oxidesemiconductor film 403 is formed over the substrate 400 with the use ofthe target. In order to remove moisture remaining in the depositionchamber, an entrapment vacuum pump such as a cryopump, an ion pump, or atitanium sublimation pump is preferably used. As an exhaustion unit, aturbo molecular pump to which a cold trap is added may be used. In thedeposition chamber which is evacuated with the cryopump, for example, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O) (further preferably, also a compound containing a carbon atom),and the like are removed, whereby the concentration of impurities in theoxide semiconductor film 403 formed in the deposition chamber can bereduced.

Further, it is preferable to successively form the gate insulating film402 and the oxide semiconductor film 403 without exposing the gateinsulating film 402 to the air. The successive formation of the gateinsulating film 402 and the oxide semiconductor film 403 withoutexposure of the gate insulating film 402 to the air can preventimpurities such as hydrogen and moisture from adsorbing onto the surfaceof the gate insulating film 402.

The oxide semiconductor film 403 can be formed by processing afilm-shaped oxide semiconductor film into an island shape by aphotolithography process.

A resist mask for forming the island-shaped oxide semiconductor film 403may be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask and thus can reduce manufacturing cost.

Note that the etching of the oxide semiconductor film may be dryetching, wet etching, or both of them. As an etchant used for wetetching of the oxide semiconductor film, for example, a mixed solutionof phosphoric acid, acetic acid, and nitric acid, or the like can beused. Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) maybe used. Further alternatively, the oxide semiconductor film may beetched by dry etching using an inductively coupled plasma (ICP) etchingmethod.

Further, heat treatment may be conducted on the oxide semiconductor film403 in order to remove excess hydrogen (including water and a hydroxylgroup) (to perform dehydration or dehydrogenation). The temperature ofthe heat treatment is higher than or equal to 300° C. and lower than orequal to 700° C., or lower than the strain point of the substrate. Theheat treatment can be conducted under reduced pressure, a nitrogenatmosphere, or the like.

In the case of using a crystalline oxide semiconductor film as the oxidesemiconductor film 403, heat treatment for crystallization may beconducted.

In this embodiment, the substrate is introduced into an electric furnacewhich is one kind of heat treatment apparatuses, and the oxidesemiconductor film 403 is subjected to heat treatment at 450° C. in anitrogen atmosphere for 1 hour and then heat treatment at 450° C. in anatmosphere including nitrogen and oxygen for 1 hour.

Note that the heat treatment apparatus used is not limited to anelectric furnace, and a device for heating a processing object by heatconduction or heat radiation from a heating element such as a resistanceheating element may alternatively be used. For example, an RTA (rapidthermal annealing) apparatus such as a GRTA apparatus or an LRTA (lamprapid thermal annealing) apparatus can be used. An LRTA apparatus is anapparatus for heating a processing object by radiation of light (anelectromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As thehigh-temperature gas, an inert gas which does not react with aprocessing object by the heat treatment, such as nitrogen or a rare gaslike argon, is used.

For example, as the heat treatment, GRTA may be conducted as follows.The substrate is put in an inert gas heated to high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out of theinert gas.

Note that in the heat treatment, it is preferable that moisture,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. The purity of nitrogen or therare gas such as helium, neon, or argon which is introduced into theheat treatment apparatus is set to preferably 6N (99.9999%) or higher,further preferably 7N (99.99999%) or higher (that is, the impurityconcentration is preferably 1 ppm or lower, further preferably 0.1 ppmor lower).

In addition, after the oxide semiconductor film 403 is heated by theheat treatment, a high-purity oxygen gas, a high-purity dinitrogenmonoxide gas, or ultra dry air (air with a moisture amount of less thanor equal to 20 ppm (−55° C. by conversion into a dew point), preferablyless than or equal to 1 ppm, or further preferably less than or equal to10 ppb, in the case where measurement is performed with use of a dewpoint meter of a cavity ring down laser spectroscopy (CRDS) system) maybe introduced into the same furnace. It is preferable that water,hydrogen, and the like be not contained in the oxygen gas or thedinitrogen monoxide gas. Alternatively, the purity of the oxygen gas orthe dinitrogen monoxide gas which is introduced into the heat treatmentapparatus is preferably 6N or higher, further preferably 7N or higher(i.e., the impurity concentration in the oxygen gas or the dinitrogenmonoxide gas is preferably 1 ppm or lower, further preferably 0.1 ppm orlower). By the effect of the oxygen gas or the dinitrogen monoxide gas,oxygen which is a main component of the oxide semiconductor and whichhas been reduced at the same time as the step of removing impurities bythe dehydration or dehydrogenation treatment is supplied, so that theoxide semiconductor film 403 can be a high-purity and electricallyi-type (intrinsic) oxide semiconductor film.

The heat treatment for dehydration or dehydrogenation may be performedafter the film-shaped oxide semiconductor film is formed, or after theisland-shaped oxide semiconductor film 403 is formed.

The heat treatment for dehydration or dehydrogenation may be performed aplurality of times and may be combined with another heat treatment.

When the heat treatment for dehydration or dehydrogenation is performedin the state where the gate insulating film 402 is covered with thefilm-shaped oxide semiconductor film which has not been processed intothe island-shaped oxide semiconductor film 403, oxygen included in thegate insulating film 402 can be prevented from being released by theheat treatment, which is preferable.

Oxygen (which includes at least one of an oxygen radical, an oxygenatom, and an oxygen ion) may be added to the dehydrated ordehydrogenated oxide semiconductor film 403 in order to supply oxygen tothe oxide semiconductor film 403.

The dehydration or dehydrogenation treatment may be accompanied withelimination of oxygen which is a main component of an oxidesemiconductor, that is, a reduction of oxygen in the oxidesemiconductor. An oxygen vacancy exists in a portion where oxygen iseliminated in the oxide semiconductor film, and a donor level whichleads to a change in the electrical characteristics of a transistor isformed owing to the oxygen vacancy.

For this reason, it is preferable to supply oxygen (which includes atleast one of an oxygen radical, an oxygen atom, and an oxygen ion) tothe dehydrated or dehydrogenated oxide semiconductor film. By supply ofoxygen to the oxide semiconductor film, the oxygen vacancy in the filmcan be filled.

The addition and supply of oxygen to the dehydrated or dehydrogenatedoxide semiconductor film 403 can increase the purity of the oxidesemiconductor film 403 and make the oxide semiconductor film 403electrically i-type (intrinsic). A transistor including the high-purityand electrically i-type (intrinsic) oxide semiconductor film 403 hassuppressed variation in electrical characteristics and is electricallystable.

Oxygen can be added by an ion implantation method, an ion doping method,a plasma immersion ion implantation method, plasma treatment, or thelike.

In the step of adding oxygen to the oxide semiconductor film 403, oxygenmay be directly added to the oxide semiconductor film 403 or to theoxide semiconductor film 403 through another film such as the insulatingfilm 407. An ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or the like may be employed for theaddition of oxygen through another film, whereas plasma treatment or thelike can be employed for the addition of oxygen directly to the exposedoxide semiconductor film 403.

The addition of oxygen to the oxide semiconductor film 403 is preferablyperformed after the dehydration or dehydrogenation treatment; however,the timing is not particularly limited. Further, oxygen may be added aplurality of times to the dehydrated or dehydrogenated oxidesemiconductor film 403.

It is preferable that the oxide semiconductor film in the transistorinclude a region that includes an excessive amount of oxygen thatexceeds the amount of oxygen in the stoichiometric composition of theoxide semiconductor in a crystalline state. In this case, the amount ofoxygen in the region exceeds that in the stoichiometric composition ofthe oxide semiconductor. Alternatively, the amount of oxygen in theregion exceeds that of the oxide semiconductor in a single crystalstate. In some cases, oxygen may exist between lattices of the oxidesemiconductor.

By removing hydrogen or moisture from the oxide semiconductor to purifythe oxide semiconductor so as not to contain impurities as much aspossible, and supplying oxygen to fill an oxygen vacancy, the oxidesemiconductor can become an i-type (intrinsic) oxide semiconductor or asubstantially i-type (intrinsic) oxide semiconductor. This enables theFermi level (E_(f)) of the oxide semiconductor to be at the same levelas the intrinsic Fermi level (E_(i)) thereof. Accordingly, by using theoxide semiconductor film in a transistor, variation in the thresholdvoltage Vth of the transistor due to an oxygen vacancy and a shift ofthe threshold voltage ΔVth can be reduced.

Next, a conductive film 445 for forming a source electrode layer and adrain electrode layer (including a wiring formed of the same layer asthe source electrode layer and the drain electrode layer) is formed overthe gate electrode layer 401, the gate insulating film 402, and theoxide semiconductor film 403 (see FIG. 1B).

The conductive film 445 is formed of a material that can withstand heattreatment performed later. As the conductive film 445 used for formingthe source electrode layer and the drain electrode layer, it is possibleto use, for example, a metal film containing an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any ofthe above-mentioned elements as its component (a titanium nitride film,a molybdenum nitride film, or a tungsten nitride film), or the like.Alternatively, it is possible to use a structure in which a film of ahigh-melting-point metal such as Ti, Mo, or W or a metal nitride filmthereof (e.g., a titanium nitride film, a molybdenum nitride film, or atungsten nitride film) is stacked over and/or below a metal film such asan Al film or a Cu film. Further alternatively, a conductive metal oxidemay be used as a material of the conductive film 445 used for formingthe source electrode layer and the drain electrode layer. As theconductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zincoxide (ZnO), indium tin oxide (In₂O₃—SnO₂; abbreviated to ITO), indiumzinc oxide (In₂O₃—ZnO), or any of these metal oxide materials containingsilicon oxide can be used.

A resist mask 448 a and a resist mask 448 b are formed over theconductive film 445 by a photolithography process. Selective etching isperformed using a gas 447 including halogen, so that the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed(see FIG. 1C). After the source electrode layer 405 a and the drainelectrode layer 405 b are formed, the resist masks 448 a and 448 b areremoved.

Ultraviolet light, a KrF laser light, an ArF laser light is preferablyused for light exposure for forming the resist masks 448 a and 448 b.The channel length L of the transistor 440 that is to be completed lateris determined by a distance between a bottom edge of the sourceelectrode layer 405 a and a bottom edge of the drain electrode layer 405b, which are adjacent to each other over the oxide semiconductor film403. In the case where the channel length L is less than 25 nm, thelight exposure at the time of forming the resist masks 448 a and 448 bis preferably performed using extreme ultraviolet light having anextremely short wavelength of several nanometers to several tens ofnanometers. In the light exposure by extreme ultraviolet light, theresolution is high and the depth of focus is large. Therefore, thechannel length (L) of the transistor to be completed later can begreater than or equal to 10 nm and less than or equal to 1000 nm,whereby operation speed of a circuit can be increased.

In order to reduce the number of photomasks used in a photolithographyprocess and reduce the number of steps, an etching step may be performedusing a resist mask formed with the use of a multi-tone mask that is alight-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed with the use of amulti-tone mask has a plurality of thicknesses and further can bechanged in shape by etching; therefore, the resist mask can be used in aplurality of etching steps for processing into different patterns.Therefore, a resist mask corresponding to at least two kinds ofdifferent patterns can be formed by one multi-tone mask. Thus, thenumber of photomasks can be reduced and the number of correspondingphotolithography steps can also be reduced, whereby simplification ofthe process can be achieved.

In this embodiment, the gas 447 including halogen is used for theetching of the conductive film 445. As the gas 447 including halogen, agas containing chlorine such as a gas containing chlorine (Cl₂), borontrichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄) can be used. As the gas 447 including halogen, agas containing fluorine such as a gas containing carbon tetrafluoride(CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), ortrifluoromethane (CHF₃) can be used. Alternatively, any of theabove-mentioned gases to which a rare gas such as helium (He) or argon(Ar) is added, or the like can be used.

As the etching method, a parallel-plate reactive ion etching (RIE)method or an ICP etching method can be used. In order to etch the filminto a desired shape, etching conditions (the amount of electric powerapplied to a coil-shaped electrode, the amount of electric power appliedto an electrode on a substrate side, the temperature of the electrode onthe substrate side, and the like) are adjusted as appropriate.

The conductive film 445 used in this embodiment is stacked layers of atitanium film with a thickness of 100 nm, an aluminum film with athickness of 400 nm, and a titanium film with a thickness of 100 nm,which are formed by a sputtering method. The conductive film 445 isetched by etching the stacked layers of the titanium film, the aluminumfilm, and the titanium film by a dry etching method; thus, the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed.

In this embodiment, two layers of the titanium film and the aluminumfilm are etched under a first etching condition, and then the othertitanium film is removed singly under a second etching condition. Notethat the first etching condition is as follows: an etching gas(BCl₃:Cl₂=750 sccm: 150 sccm) is used, the bias power is 1500 W, the ICPpower supply is 0 W, and the pressure is 2.0 Pa. The second etchingcondition is as follows: an etching gas (BCl₃:Cl₂=700 sccm: 100 sccm) isused, the bias power is 750 W, the ICP power supply is 0 W, and thepressure is 2.0 Pa.

In the etching step for forming the source electrode layer 405 a and thedrain electrode layer 405 b, a residue of an etchant (an etching gas oran etching solution) remains on a surface of the oxide semiconductorfilm and in the vicinity thereof. This residue causes a reduction orvariation in electrical characteristics of the transistor such as anincrease in leakage current. Further, an element contained in theetchant may enter or attach to the oxide semiconductor film 403 andadversely affect transistor characteristics.

The residue includes the etchant (the etching gas or the etchingsolution), the processed conductive film 445, the element contained inthe oxide semiconductor film 403 which is exposed to the etchant, and acompound of such an element. For example, a gas including halogen isfavorably used in the etching step for forming the source and drainelectrode layers; in that case, the residue includes a halogen impurity(halogen or a halide).

As the residue, chlorine, fluorine, boron, phosphorus, aluminum, iron,carbon, or the like can be given, for example. In addition, the residuemay include the conductive film 445, a metal element (e.g., indium,gallium, or zinc) included in the oxide semiconductor film 403 which isexposed to the etchant, a halide of the metal element, an oxide of themetal element, or the like, in some cases. Further, an element includedin the resist masks 448 a and 448 b may also be included as the residuein some cases.

In this embodiment, because the gas 447 including halogen is used in theetching step for forming the source electrode layer 405 a and the drainelectrode layer 405 b, the remaining residue is a halogen (in thisembodiment, chlorine) impurity (halogen or a halide). Further, in thecase where boron is also included in the gas 447 including halogen as inthis embodiment, the remaining residue includes boron or a compoundincluding boron. Furthermore, in the case where a mixed solution ofphosphoric acid, acetic acid, and nitric acid is used as the etchant,the residue includes phosphorus or the like.

After the source electrode layer 405 a and the drain electrode layer 405b are formed, a step of removing the residue existing on the surface ofthe oxide semiconductor film 403 and in the vicinity of the surfacebetween the source electrode layer 405 a and the drain electrode layer405 b is performed (see FIG. 1D). Treatment using water or an alkalinesolution or plasma treatment can be performed as the residue removalstep. For example, treatment using water or a TMAH solution, plasmatreatment using oxygen, dinitrogen monoxide, or a rare gas (typicallyargon), or the like can be favorably used. Alternatively, treatmentusing dilute hydrofluoric acid may be used. The step of removing theresidue has an effect of removing the residue (in this embodiment,mainly halogen or a halide) attached onto the surface of the oxidesemiconductor film 403.

It is preferable that etching conditions be optimized so as not to etchand cut the oxide semiconductor film 403 in the step of etching theconductive film 445 and in the residue removal step. However, it isdifficult to obtain etching conditions under which only the conductivefilm 445 is etched and the oxide semiconductor film 445 is not etched atall. In some cases, part of the oxide semiconductor film 403 is etchedwhen the conductive film 445 is etched, so that an oxide semiconductorfilm having a groove portion (a recessed portion) is formed.

Through the above-described process, the transistor 440 of thisembodiment is manufactured.

In this embodiment, the insulating film 407 serving as a protectiveinsulating film is formed in contact with the oxide semiconductor film403 over the source electrode layer 405 a and the drain electrode layer405 b (see FIG. 1E).

The insulating film 407 has a thickness of at least 1 nm and can beformed by a method by which impurities such as water and hydrogen do notenter the insulating film 407, such as a sputtering method, asappropriate. When hydrogen is contained in the insulating film 407,entry of the hydrogen to the oxide semiconductor film 403, or extractionof oxygen from the oxide semiconductor film by hydrogen may occur, inwhich case the resistance of the back channel in the oxide semiconductorfilm 403 may be decreased (the back channel may have n-typeconductivity), so that a parasitic channel may be formed. Therefore, itis important that a film formation method in which hydrogen is not usedis employed in order to form the insulating film 407 containing aslittle hydrogen as possible.

As the insulating film 407, a single layer or stacked layers of aninorganic insulating film typified by a silicon oxide film, a siliconoxynitride film, an aluminum oxide film, an aluminum oxynitride film, ahafnium oxide film, a gallium oxide film, a silicon nitride film, analuminum nitride film, a silicon nitride oxide film, an aluminum nitrideoxide film, and the like can be used.

In the case of performing a heating step as the dehydration ordehydrogenation treatment, it is preferable to supply oxygen to thedehydrated or dehydrogenated oxide semiconductor film 403. By supply ofoxygen to the oxide semiconductor film 403, an oxygen vacancy in thefilm can be filled.

In this embodiment, oxygen is supplied to the oxide semiconductor film403 using the insulating film 407 as a supply source, and thus anexample in which an oxide insulating film (e.g., a silicon oxide film ora silicon oxynitride film) including oxygen is used as the insulatingfilm 407 is described. In the case where the insulating film 407 is usedas a supply source of oxygen, the insulating film 407 can favorablyfunction as the supply source of oxygen when being a film including much(an excessive amount of) oxygen (preferably, a film including a regionthat includes an excessive amount of oxygen that exceeds the amount ofoxygen in the stoichiometric composition of the oxide semiconductor in acrystalline state).

In this embodiment, as the insulating film 407, a silicon oxide filmhaving a thickness of 300 nm is formed by a sputtering method. Thesubstrate temperature in the film deposition may be higher than or equalto room temperature and lower than or equal to 300° C. and in thisembodiment, is 100° C. The silicon oxide film can be formed by asputtering method under a rare gas (typically, argon) atmosphere, anoxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen. As atarget, a silicon oxide target or a silicon target can be used. Forexample, with use of a silicon target, a silicon oxide film can beformed by a sputtering method under an atmosphere including oxygen.

In order to remove residual moisture from the deposition chamber of theinsulating film 407 as in the deposition of the oxide semiconductor film403, an entrapment vacuum pump (such as a cryopump) is preferably used.When the insulating film 407 is deposited in the deposition chamberevacuated using a cryopump, the impurity concentration of the insulatingfilm 407 can be reduced. As an evacuation unit for removing residualmoisture in the deposition chamber of the insulating film 407, a turbomolecular pump provided with a cold trap may be used.

It is preferable to use a high-purity gas from which an impurity such ashydrogen or water is removed as a sputtering gas for the formation ofthe insulating film 407.

Next, the oxide semiconductor film 403 is subjected to a heating step ina state in which part of the oxide semiconductor film 403 (a channelformation region) is in contact with the insulating film 407.

The heating step is performed at a temperature higher than or equal to250° C. and lower than or equal to 700° C., preferably higher than orequal to 400° C. and lower than or equal to 700° C., or lower than thestrain point of the substrate. For example, the substrate is introducedinto an electric furnace which is one kind of heat treatmentapparatuses, and the heating step is performed on the oxidesemiconductor film at 250° C. for one hour in a nitrogen atmosphere.

For the heating step, a heating method and a heating apparatus similarto those for the heating step for the dehydration or dehydrogenationtreatment can be used.

The heating step may be performed under reduced pressure or in anitrogen atmosphere, an oxygen atmosphere, ultra dry air (air with amoisture amount of less than or equal to 20 ppm (−55° C. by conversioninto a dew point), preferably less than or equal to 1 ppm, or furtherpreferably less than or equal to 10 ppb, in the case where measurementis performed with use of a dew point meter of a cavity ring down laserspectroscopy (CRDS) system), or a rare gas (argon, helium, or the like)atmosphere. It is preferable that water, hydrogen, and the like be notcontained in the nitrogen atmosphere, the oxygen atmosphere, the ultradry air, the rare gas atmosphere, or the like. It is also preferablethat the purity of nitrogen, oxygen, or the rare gas which is introducedinto the heat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

The oxide semiconductor film 403 and the insulating film 407 includingoxygen are in contact with each other when the heating step isperformed; thus, oxygen which is one of the main components of the oxidesemiconductor film 403 and which has been reduced at the same time asthe step of removing impurities can be supplied from the insulating film407 including oxygen to the oxide semiconductor film 403.

Furthermore, a highly dense inorganic insulating film may be providedover the insulating film 407. For example, an aluminum oxide film isformed over the insulating film 407 by a sputtering method. The highlydense aluminum oxide film (with a film density of 3.2 g/cm³ or more,preferably 3.6 g/cm³ or more) enables stable electrical characteristicsof the transistor 440. The film density can be measured by Rutherfordbackscattering spectrometry (RBS) or X-ray reflection (XRR).

An aluminum oxide film which can be used as the protective insulatingfilm provided over the transistor 440 has a high shielding effect(blocking effect) of preventing penetration of both oxygen and animpurity such as hydrogen or moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm functions as a protective film for preventing entry of an impuritysuch as hydrogen or moisture, which causes a change, into the oxidesemiconductor film 403 and release of oxygen, which is a mainconstituent material of the oxide semiconductor, from the oxidesemiconductor film 403.

In addition, a planarization insulating film may be formed in order toreduce surface unevenness due to the transistor 440. As theplanarization insulating film, an organic material such as polyimide, anacrylic resin, or a benzocyclobutene-based resin can be used. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material) or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

For example, an acrylic resin film with a thickness of 1500 nm may beformed as the planarization insulating film. The acrylic resin film canbe formed by coating using a coating method and then baking (e.g., at250° C. under a nitrogen atmosphere for 1 hour).

After the formation of the planarization insulating film, heat treatmentmay be performed. For example, heat treatment is performed at 250° C.under a nitrogen atmosphere for 1 hour.

In this manner, after the formation of the transistor 440, heattreatment may be performed. The heat treatment may be performed aplurality of times.

Since the surface of the oxide semiconductor film 403 and the vicinityof the surface can be prevented from being contaminated by the residueby performing the above-described step of removing the residue, in thesemiconductor device including the transistor 440 that is an invertedstaggered bottom-gate transistor, the surface density of the impuritiesdue to the etching step (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of theoxide semiconductor film 403 can be 1×10¹³ atoms/cm² or lower(preferably 1×10¹² atoms/cm² or lower). Further, the concentration ofthe impurities due to the etching step (typically halogen (e.g.,chlorine, fluorine), boron, phosphorus, aluminum, iron, or carbon) onthe surface of the oxide semiconductor film 403 can be 5×10¹⁸ atoms/cm³or lower (preferably 1×10¹⁸ atoms/cm³ or lower).

Note that the concentration of the impurities due to the etching step(typically halogen (e.g., chlorine, fluorine), boron, phosphorus,aluminum, iron, or carbon) can be estimated by a method such as SIMS(secondary ion mass spectrometry).

Accordingly, a highly reliable semiconductor device including thetransistor 440 using the oxide semiconductor film 403 and having stableelectrical characteristics can be provided. In addition, the highlyreliable semiconductor device can be manufactured with high yield, sothat productivity can be improved.

Embodiment 2

In this embodiment, an embodiment of a semiconductor device and a methodfor manufacturing the semiconductor device is described with referenceto FIGS. 2A to 2E. The same portion as or a portion having a similarfunction to those in the above embodiment can be formed in a mannersimilar to that described in the above embodiment, and also the stepssimilar to those in the above embodiment can be performed in a mannersimilar to that described in the above embodiment, and repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

A transistor 430 illustrated in FIG. 2E is an example of an invertedstaggered transistor that is one type of a bottom-gate transistor. Notethat FIGS. 2A to 2E are cross-sectional views taken along a channellength direction of the transistor 430.

As illustrated in FIG. 2E, a semiconductor device including thetransistor 430 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating film 402, theoxide semiconductor film 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. The insulating film 407 is provided tocover the transistor 430.

An example of a method for manufacturing the semiconductor deviceincluding the transistor 430 is illustrated in FIGS. 2A to 2E.

A conductive film 441 is formed over the substrate 400 (see FIG. 2A).The conductive film 441 can be formed using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material which contains any of thesemetal materials as its main component. Alternatively, a semiconductorfilm typified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the conductive film 441. The conductive film 441 mayhave a single-layer structure or a stacked-layer structure.

The conductive film 441 can also be formed using a conductive materialsuch as indium oxide including tungsten oxide, indium oxide includingtitanium oxide, indium tin oxide, indium tin oxide including titaniumoxide, indium tin oxide to which silicon oxide is added, indium zincoxide, or indium zinc oxide including tungsten oxide. It is alsopossible that the conductive film 441 has a stacked-layer structure ofthe above conductive material and the above metal material.

In this embodiment, a tungsten film with a thickness of 100 nm is formedby a sputtering method as the conductive film 441.

A resist mask 442 is formed over the conductive film 441 by aphotolithography process, and selective etching is performed to form thegate electrode layer 401 (see FIG. 2B). After the gate electrode layer401 is formed, the resist mask 442 is removed. The etching of theconductive film 441 may be dry etching, wet etching, or both of them.

In this embodiment, the gas 443 including halogen is used for theetching of the conductive film 441. As the gas 443 including halogen, agas containing chlorine such as a gas containing chlorine (Cl₂), borontrichloride (BCl₃), silicon tetrachloride (SiCl₄), or carbontetrachloride (CCl₄) can be used. As the gas 443 including halogen, agas containing fluorine such as a gas containing carbon tetrafluoride(CF₄), sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), ortrifluoromethane (CHF₃) can be used. Alternatively, any of theabove-mentioned gases to which a rare gas such as helium (He) or argon(Ar) is added, or the like can be used.

As the etching method, a parallel-plate RIE method or an ICP etchingmethod can be used. In order to etch the film into a desired shape,etching conditions (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, and the like) are adjusted as appropriate.

In this embodiment, dry etching is used in the etching step of theconductive film 441. As the gas 443 including halogen, a gas includingcarbon tetrafluoride, chlorine, and oxygen (CF₄:Cl₂:O₂=25 sccm: 25 sccm:10 sccm) is used. The bias power is 1500 W, the ICP power supply is 500W, and the pressure is 1.0 Pa.

In the etching step for forming the gate electrode layer 401, a residueof an etchant (an etching gas or an etching solution) remains on asurface of the gate electrode layer 401 and in the vicinity thereof. Ifimpurities included in the residue exist on the surface of the gateelectrode layer, resistance to voltage (withstand voltage) of the gateinsulating film 402 is decreased and leakage current between the gateelectrode layer 401 and the source electrode layer 405 a or the drainelectrode layer 405 b is caused. This causes a reduction or variation inelectrical characteristics of the transistor.

The residue includes the etchant (the etching gas or the etchingsolution), the element contained in the processed conductive film 441,and a compound of such an element. For example, a gas including halogenis favorably used in the etching step for forming the gate electrodelayer 401; in that case, the residue includes a halogen impurity(halogen or a halide).

As the residue, chlorine, fluorine, boron, phosphorus, aluminum, iron,carbon, or the like can be given, for example. In addition, the residuemay include a metal element included in the conductive film 441, ahalide of the metal element, an oxide of the metal element, or the like,in some cases. Further, an element included in the resist mask 442 mayalso be included as the residue in some cases.

In this embodiment, because the gas 443 including halogen is used in theetching step for forming the gate electrode layer 401, the remainingresidue is a halogen (in this embodiment, chlorine) impurity (halogen ora halide). Furthermore, in the case where a mixed solution of phosphoricacid, acetic acid, and nitric acid is used as the etchant, the residueincludes phosphorus or the like.

After the gate electrode layer 401 is formed, a step of removing theresidue existing on the surface of the gate electrode layer 401 and inthe vicinity of the surface is performed (see FIG. 2C). Treatment usingwater or an alkaline solution or plasma treatment can be performed asthe residue removal step. For example, treatment using water or a TMAHsolution, plasma treatment using oxygen, dinitrogen monoxide, or a raregas (typically argon), or the like can be favorably used. Alternatively,treatment using dilute hydrofluoric acid may be used.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401. In this embodiment, a silicon oxynitride film with athickness of 200 nm is formed by a high-density plasma CVD method.

The oxide semiconductor film 403 is formed over the gate insulating film402 (see FIG. 2D). In this embodiment, as the oxide semiconductor film403, an In—Ga—Zn-based oxide film (an IGZO film) is formed with athickness of 35 nm by a sputtering method using a sputtering apparatusthat includes an AC power supply device. In this embodiment, anIn—Ga—Zn-based oxide target with an atomic ratio of In:Ga:Zn=1:1:1(=1/3:1/3:1/3) is used. The deposition conditions are as follows: theatmosphere is oxygen and argon (the flow rate of oxygen: 50%), thepressure is 0.6 Pa, the power supply is 5 kW, and the substratetemperature is 170° C. The deposition rate under these depositionconditions is 16 nm/min.

Heat treatment may be conducted on the oxide semiconductor film 403 inorder to remove excess hydrogen (including water and a hydroxyl group)(to perform dehydration or dehydrogenation). In this embodiment, thesubstrate is introduced into an electric furnace which is one kind ofheat treatment apparatuses, and the oxide semiconductor film 403 issubjected to heat treatment at 450° C. in a nitrogen atmosphere for 1hour and then heat treatment at 450° C. in an atmosphere includingnitrogen and oxygen for 1 hour.

Next, a conductive film is formed over the gate electrode layer 401, thegate insulating film 402, and the oxide semiconductor film 403 and isetched, so that the source electrode layer 405 a and the drain electrodelayer 405 b are formed. Note that the etching of the conductive film maybe performed by dry etching, wet etching, or both of them.

In this embodiment, a titanium film with a thickness of 100 nm, analuminum film with a thickness of 400 nm, and a titanium film with athickness of 100 nm are stacked by a sputtering method, and the stackedlayers of the titanium film, the aluminum film, and the titanium filmare etched by a dry etching method, so that the source electrode layer405 a and the drain electrode layer 405 b are formed.

Through the above-described process, the transistor 430 of thisembodiment is manufactured.

In this embodiment, the insulating film 407 serving as a protectiveinsulating film is formed in contact with the oxide semiconductor film403 over the source electrode layer 405 a and the drain electrode layer405 b (see FIG. 2E). For example, a silicon oxynitride film with athickness of 400 nm is formed by a CVD method. After the formation ofthe protective insulating film, heat treatment may be performed. Forexample, heat treatment is performed at 300° C. in a nitrogen atmospherefor 1 hour.

In addition, a planarization insulating film may be formed in order toreduce surface unevenness due to the transistor 430.

For example, an acrylic resin film with a thickness of 1500 nm may beformed as the planarization insulating film over the protectiveinsulating film. The acrylic resin film can be formed by coating using acoating method and then baking (e.g., at 250° C. under a nitrogenatmosphere for 1 hour).

After the formation of the planarization insulating film, heat treatmentmay be performed. For example, heat treatment is performed at 250° C.under a nitrogen atmosphere for 1 hour.

Since the surface of the gate electrode layer 401 and the vicinity ofthe surface can be prevented from being contaminated by the residue, inthe semiconductor device including the transistor 420 that is aninverted staggered bottom-gate transistor, the surface density of theimpurities due to the etching step (typically halogen (e.g., chlorine,fluorine), boron, phosphorus, aluminum, iron, or carbon) on the surfaceof the gate electrode layer 401 is 1×10¹³ atoms/cm² or lower (preferably1×10¹² atoms/cm² or lower). Further, the concentration of the impuritiesdue to the etching step (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of the gateelectrode layer 401 can be 5×10¹⁸ atoms/cm³ or lower (preferably 1×10¹⁸atoms/cm³ or lower).

Accordingly, a highly reliable semiconductor device including thetransistor 430 using the oxide semiconductor film 403 and having stableelectrical characteristics can be provided. In addition, the highlyreliable semiconductor device can be manufactured with high yield, sothat productivity can be improved.

Embodiment 3

In this embodiment, an embodiment of a semiconductor device and a methodfor manufacturing the semiconductor device is described with referenceto FIGS. 3A to 3F. The same portion as or a portion having a similarfunction to those in the above embodiments can be formed in a mannersimilar to that described in the above embodiments, and also the stepssimilar to those in the above embodiments can be performed in a mannersimilar to that described in the above embodiments, and repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

A transistor 420 illustrated in FIG. 3F is an example of an invertedstaggered transistor that is one type of a bottom-gate transistor. Notethat FIGS. 3A to 3F are cross-sectional views taken along a channellength direction of the transistor 420.

As illustrated in FIG. 3F, a semiconductor device including thetransistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating film 402, theoxide semiconductor film 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. The insulating film 407 is provided tocover the transistor 420.

An example of a method for manufacturing the semiconductor deviceincluding the transistor 420 is illustrated in FIGS. 3A to 3F.

A conductive film is formed over the substrate 400 having an insulatingsurface. The conductive film can be formed using a metal material suchas molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material which contains any of thesemetal materials as its main component. Alternatively, a semiconductorfilm typified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the conductive film. The conductive film may have asingle-layer structure or a stacked-layer structure.

The conductive film can also be formed using a conductive material suchas indium oxide including tungsten oxide, indium oxide includingtitanium oxide, indium tin oxide, indium tin oxide including titaniumoxide, indium tin oxide to which silicon oxide is added, indium zincoxide, or indium zinc oxide including tungsten oxide. It is alsopossible that the conductive film has a stacked-layer structure of theabove conductive material and the above metal material.

In this embodiment, a tungsten film with a thickness of 100 nm is formedby a sputtering method as the conductive film.

A resist mask is formed over the conductive film by a photolithographyprocess, and selective etching is performed to form the gate electrodelayer 401 (see FIG. 3A). After the gate electrode layer 401 is formed,the resist mask is removed. The etching of the conductive film may bedry etching, wet etching, or both of them.

In this embodiment, the gas including halogen is used for the etching ofthe conductive film. As the gas including halogen, a gas containingchlorine such as a gas containing chlorine (Cl₂), boron trichloride(BCl₃), silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)can be used. As the gas including halogen, a gas containing fluorinesuch as a gas containing carbon tetrafluoride (CF₄), sulfur hexafluoride(SF₆), nitrogen trifluoride (NF₃), or trifluoromethane (CHF₃) can beused. Alternatively, any of the above-mentioned gases to which a raregas such as helium (He) or argon (Ar) is added, or the like can be used.

As the etching method, a parallel-plate RIE method or an ICP etchingmethod can be used. In order to etch the film into a desired shape,etching conditions (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, and the like) are adjusted as appropriate.

In this embodiment, dry etching is used in the etching step of theconductive film. Etching conditions are as follows: as the gas includinghalogen, a gas including carbon tetrafluoride, chlorine, and oxygen(CF₄:Cl₂:O₂=25 sccm: 25 sccm: 10 sccm) is used; the bias power is 1500W; the ICP power supply is 500 W; and the pressure is 1.0 Pa.

In the etching step for forming the gate electrode layer 401, a residueof an etchant (an etching gas or an etching solution) remains on asurface of the gate electrode layer 401 and in the vicinity thereof. Ifimpurities included in the residue exist on the surface of the gateelectrode layer, withstand voltage of the gate insulating film 402 isdecreased and leakage current between the gate electrode layer 401 andthe source electrode layer 405 a or the drain electrode layer 405 b iscaused. This causes a reduction or variation in electricalcharacteristics of the transistor.

The residue includes the etchant (the etching gas or the etchingsolution), the element contained in the processed conductive film, and acompound of such an element. For example, a gas including halogen isfavorably used in the etching step for forming the gate electrode layer401; in that case, the residue includes a halogen impurity (halogen or ahalide).

As the residue, chlorine, fluorine, boron, phosphorus, aluminum, iron,carbon, or the like can be given, for example. In addition, the residuemay include a metal element included in the conductive film, a halide ofthe metal element, an oxide of the metal element, or the like, in somecases. Further, an element included in the resist mask may also beincluded as the residue in some cases.

In this embodiment, because the gas including halogen is used in theetching step for forming the gate electrode layer 401, the remainingresidue is a halogen (in this embodiment, chlorine) impurity (halogen ora halide). Furthermore, in the case where a mixed solution of phosphoricacid, acetic acid, and nitric acid is used as the etchant, the residueincludes phosphorus or the like.

After the gate electrode layer 401 is formed, a step of removing theresidue existing on the surface of the gate electrode layer 401 and inthe vicinity of the surface is performed (see FIG. 3B). Treatment usingwater or an alkaline solution or plasma treatment can be performed asthe residue removal step. For example, treatment using water or a TMAHsolution, plasma treatment using oxygen, dinitrogen monoxide, or a raregas (typically argon), or the like can be favorably used. Alternatively,treatment using dilute hydrofluoric acid may be used.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401. In this embodiment, a silicon oxynitride film with athickness of 200 nm is formed by a high-density plasma CVD method.

The oxide semiconductor film 403 is formed over the gate insulating film402 (see FIG. 3C). In this embodiment, as the oxide semiconductor film403, an In—Ga—Zn-based oxide film (an IGZO film) is formed with athickness of 35 nm by a sputtering method using a sputtering apparatusthat includes an AC power supply device. In this embodiment, anIn—Ga—Zn-based oxide target with an atomic ratio of In:Ga:Zn=1:1:1(=1/3:1/3:1/3) is used. The deposition conditions are as follows: theatmosphere is oxygen and argon (the flow rate of oxygen: 50%), thepressure is 0.6 Pa, the power supply is 5 kW, and the substratetemperature is 170° C. The deposition rate under these depositionconditions is 16 nm/min.

Heat treatment may be conducted on the oxide semiconductor film 403 inorder to remove excess hydrogen (including water and a hydroxyl group)(to perform dehydration or dehydrogenation). In this embodiment, thesubstrate is introduced into an electric furnace which is one kind ofheat treatment apparatuses, and the oxide semiconductor film 403 issubjected to heat treatment at 450° C. in a nitrogen atmosphere for 1hour and then heat treatment at 450° C. in an atmosphere includingnitrogen and oxygen for 1 hour.

Next, a conductive film to be processed into a source electrode layerand a drain electrode layer is formed over the gate electrode layer 401,the gate insulating film 402, and the oxide semiconductor film 403.

The conductive film is formed of a material that can withstand heattreatment performed later. As the conductive film used for forming thesource electrode layer and the drain electrode layer, it is possible touse, for example, a metal film containing an element selected from Al,Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any of theabove-mentioned elements as its component (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film), or the like.Alternatively, it is possible to use a structure in which a film of ahigh-melting-point metal such as Ti, Mo, or W or a metal nitride filmthereof (e.g., a titanium nitride film, a molybdenum nitride film, or atungsten nitride film) is stacked over and/or below a metal film such asan Al film or a Cu film. Further alternatively, a conductive metal oxidemay be used as a material of the conductive film used for forming thesource electrode layer and the drain electrode layer. As the conductivemetal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO),indium tin oxide (In₂O₃—SnO₂; abbreviated to ITO), indium zinc oxide(In₂O₃—ZnO), or any of these metal oxide materials containing siliconoxide can be used.

A resist mask is formed over the conductive film by a photolithographyprocess. Selective etching is performed, so that the source electrodelayer 405 a and the drain electrode layer 405 b are formed (see FIG.3D). After the source electrode layer 405 a and the drain electrodelayer 405 b are formed, the resist mask is removed.

In this embodiment, the gas including halogen is used for the etching ofthe conductive film. As the gas including halogen, a gas containingchlorine such as a gas containing chlorine (Cl₂), boron trichloride(BCl₃), silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)can be used. As the gas including halogen, a gas containing fluorinesuch as a gas containing carbon tetrafluoride (CF₄), sulfur hexafluoride(SF₆), nitrogen trifluoride (NF₃), or trifluoromethane (CHF₃) can beused. Alternatively, any of the above-mentioned gases to which a raregas such as helium (He) or argon (Ar) is added, or the like can be used.

As the etching method, a parallel-plate RIE method or an ICP etchingmethod can be used. In order to etch the film into a desired shape,etching conditions (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, and the like) are adjusted as appropriate.

The conductive film used in this embodiment is stacked layers of atitanium film with a thickness of 100 nm, an aluminum film with athickness of 400 nm, and a titanium film with a thickness of 100 nm,which are formed by a sputtering method. The conductive film is etchedby etching the stacked layers of the titanium film, the aluminum film,and the titanium film by a dry etching method; thus, the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed.

In this embodiment, two layers of the titanium film and the aluminumfilm are etched under a first etching condition, and then the othertitanium film is removed singly under a second etching condition. Notethat the first etching condition is as follows: an etching gas(BCl₃:Cl₂=750 sccm: 150 sccm) is used, the bias power is 1500 W, the ICPpower supply is 0 W, and the pressure is 2.0 Pa. The second etchingcondition is as follows: an etching gas (BCl₃:Cl₂=700 sccm: 100 sccm) isused, the bias power is 750 W, the ICP power supply is 0 W, and thepressure is 2.0 Pa.

In the etching step for forming the source electrode layer 405 a and thedrain electrode layer 405 b, a residue of an etchant (an etching gas oran etching solution) remains on a surface of the oxide semiconductorfilm and in the vicinity thereof. This residue causes a reduction orvariation in electrical characteristics of the transistor such as anincrease in leakage current. Further, an element contained in theetchant may enter or attach to the oxide semiconductor film 403 andadversely affect transistor characteristics.

The residue includes the etchant (the etching gas or the etchingsolution), the processed conductive film, the element contained in theoxide semiconductor film 403 which is exposed to the etchant, and acompound of such an element. For example, a gas including halogen isfavorably used in the etching step for forming the source and drainelectrode layers; in that case, the residue includes a halogen impurity(halogen or a halide).

As the residue, chlorine, fluorine, boron, phosphorus, aluminum, iron,carbon, or the like can be given, for example. In addition, the residuemay include the conductive film, a metal element (e.g., indium, gallium,or zinc) included in the oxide semiconductor film 403 which is exposedto the etchant, a halide of the metal element, an oxide of the metalelement, or the like, in some cases. Further, an element included in theresist mask may also be included as the residue in some cases.

In this embodiment, because the gas including halogen is used in theetching step for forming the source electrode layer 405 a and the drainelectrode layer 405 b, the remaining residue is a halogen (in thisembodiment, chlorine) impurity (halogen or a halide). Further, in thecase where boron is also included in the gas including halogen as inthis embodiment, the remaining residue includes boron or a compoundincluding boron. Furthermore, in the case where a mixed solution ofphosphoric acid, acetic acid, and nitric acid is used as the etchant,the residue includes phosphorus or the like.

After the source electrode layer 405 a and the drain electrode layer 405b are formed, a step of removing the residue existing on the surface ofthe oxide semiconductor film 403 and in the vicinity of the surfacebetween the source electrode layer 405 a and the drain electrode layer405 b is performed (see FIG. 3E). Treatment using water or an alkalinesolution or plasma treatment can be performed as the residue removalstep. For example, treatment using water or a TMAH solution, plasmatreatment using oxygen, dinitrogen monoxide, or a rare gas (typicallyargon), or the like can be favorably used. Alternatively, treatmentusing dilute hydrofluoric acid may be used. The step of removing theresidue has an effect of removing the residue (in this embodiment,mainly halogen or a halide) attached onto the surface of the oxidesemiconductor film 403.

Through the above-described process, the transistor 420 of thisembodiment is manufactured.

In this embodiment, the insulating film 407 serving as a protectiveinsulating film is formed in contact with the oxide semiconductor film403 over the source electrode layer 405 a and the drain electrode layer405 b (see FIG. 3F). For example, a silicon oxynitride film with athickness of 400 nm is formed by a CVD method. After the formation ofthe protective insulating film, heat treatment may be performed. Forexample, heat treatment is performed at 300° C. in a nitrogen atmospherefor 1 hour.

In addition, a planarization insulating film may be formed in order toreduce surface unevenness due to the transistor 430.

For example, an acrylic resin film with a thickness of 1500 nm may beformed as the planarization insulating film over the protectiveinsulating film. The acrylic resin film can be formed by coating using acoating method and then baking (e.g., at 250° C. under a nitrogenatmosphere for 1 hour).

After the formation of the planarization insulating film, heat treatmentmay be performed. For example, heat treatment is performed at 250° C.under a nitrogen atmosphere for 1 hour.

Since the surfaces of the gate electrode layer 401 and the oxidesemiconductor film 403 and the vicinity of the surfaces can be preventedfrom being contaminated by the residue by the above-described method, inthe semiconductor device including the transistor 420 that is aninverted staggered bottom-gate transistor, the surface density of theimpurities due to the etching step (typically halogen (e.g., chlorine,fluorine), boron, phosphorus, aluminum, iron, or carbon) on the surfaceof the oxide semiconductor film 403 can be 1×10¹³ atoms/cm² or lower(preferably 1×10¹² atoms/cm² or lower). Further, the surface density ofthe impurities due to the etching step (typically halogen (e.g.,chlorine, fluorine), boron, phosphorus, aluminum, iron, or carbon) onthe surface of the gate electrode layer 401 can be 1×10¹³ atoms/cm² orlower (preferably 1×10¹² atoms/cm² or lower).

The concentration of the impurities due to the etching step (typicallyhalogen (e.g., chlorine, fluorine), boron, phosphorus, aluminum, iron,or carbon) on the surface of the oxide semiconductor film 403 can be5×10¹⁸ atoms/cm³ or lower (preferably 1×10¹⁸ atoms/cm³ or lower).Further, the concentration of the impurities due to the etching step(typically halogen (e.g., chlorine, fluorine), boron, phosphorus,aluminum, iron, or carbon) on the surface of the gate electrode layer401 can be 5×10¹⁸ atoms/cm³ or lower (preferably 1×10¹⁸ atoms/cm³ orlower).

Accordingly, a highly reliable semiconductor device including thetransistor 420 using the oxide semiconductor film 403 and having stableelectrical characteristics can be provided. In addition, the highlyreliable semiconductor device can be manufactured with high yield, sothat productivity can be improved.

Embodiment 4

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using any of the transistorsdescribed in Embodiments 1 to 3. Further, part or all of the drivercircuitry which includes the transistor can be formed over a substratewhere a pixel portion is formed, whereby a system-on-panel can beformed.

In FIG. 4A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed with a second substrate 4006. In FIG. 4A, a scan line drivercircuit 4004 and a signal line driver circuit 4003 which are each formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared are mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. A variety of signals and potentials aresupplied to the signal line driver circuit 4003 and the scan line drivercircuit 4004 each of which is separately formed, and the pixel portion4002 from flexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 4B and 4C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with a display element by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 4B and 4C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 4B and 4C, a variety of signalsand potentials are supplied to the signal line driver circuit 4003 whichis separately formed, the scan line driver circuit 4004, and the pixelportion 4002 from the FPC 4018.

Although FIGS. 4B and 4C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, one embodiment of the present invention is not limitedto this structure. The scan line driver circuit may be formed separatelyand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be formed separately and thenmounted.

Note that there is no particular limitation on a connection method of aseparately formed driver circuit, and a chip on glass (COG) method, awire bonding method, a tape automated bonding (TAB) method, or the likecan be used. FIG. 4A illustrates an example in which the signal linedriver circuit 4003 and the scan line driver circuit 4004 are mounted bya COG method. FIG. 4B illustrates an example in which the signal linedriver circuit 4003 is mounted by a COG method. FIG. 4C illustrates anexample in which the signal line driver circuit 4003 is mounted by a TABmethod.

The display device includes, in its category, a panel in which a displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice).

Further, the display device also includes the following modules in itscategory: a module to which a connector such as an FPC, a TAB tape, or aTCP is attached; a module having a TAB tape or a TCP at the tip of whicha printed wiring board is provided; and a module in which an integratedcircuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors, and the transistordescribed in any of Embodiments 1 to 3 can be applied thereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with referenceto FIGS. 4A to 4C, FIGS. 5A and 5B, and FIGS. 6A and 6B. FIGS. 6A and 6Bcorrespond to the cross-sectional view taken along line M-N in FIG. 4B.

As illustrated in FIGS. 4A to 4C and FIGS. 6A and 6B, the semiconductordevice includes a connection terminal electrode 4015 and a terminalelectrode 4016. The connection terminal electrode 4015 and the terminalelectrode 4016 are electrically connected to a terminal included in theFPC 4018 (FPCs 4018 a and 4018 b) through an anisotropic conductive film4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as gateelectrode layers of the transistors 4010 and 4011.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. FIGS. 6A and 6B illustrate the transistor 4010 included inthe pixel portion 4002 and the transistor 4011 included in the scan linedriver circuit 4004. In FIG. 6A, an insulating film 4020 is providedover the transistors 4010 and 4011, and in FIG. 6B, an insulating film4021 is further provided.

The transistor described in any of Embodiments 1 to 3 can be applied tothe transistor 4010 and the transistor 4011. In this embodiment, anexample in which a transistor whose structure and manufacturing methodare similar to those of the transistor 440 described in Embodiment 1 isused will be described.

In manufacturing the transistors 4010 and 4011 whose structure andmanufacturing method are similar to those of the transistor 440described in Embodiment 1, after the source electrode layer and thedrain electrode layer are formed, a step of removing the residueexisting on the surface of the oxide semiconductor film and in thevicinity of the surface between the source electrode layer and the drainelectrode layer is performed. Treatment using water or an alkalinesolution or plasma treatment can be performed as the residue removalstep. For example, treatment using water or a TMAH solution, plasmatreatment using oxygen, dinitrogen monoxide, or a rare gas (typicallyargon), or the like can be favorably used. Alternatively, treatmentusing dilute hydrofluoric acid may be used.

As described in Embodiment 2, after the formation of the gate electrodelayer, the step of removing the residue caused on the surface of thegate electrode layer and in the vicinity of the surface due to theetching step may be performed. Further, as described in Embodiment 3,the step of removing the residue on the surface of the gate electrodelayer and in the vicinity of the surface may be performed after theformation of the gate electrode layer, and then the step of removing theresidue on the surface of the oxide semiconductor film and in thevicinity of the surface may be performed after the formation of thesource and drain electrode layers.

Since the surface of the oxide semiconductor film and the vicinity ofthe surface can be prevented from being contaminated by the residue, inthe transistors 4010 and 4011, the surface density of the impurities dueto the etching step (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of theoxide semiconductor film can be 1×10¹³ atoms/cm² or lower (preferably1×10¹² atoms/cm² or lower). Further, the concentration of the impuritiesdue to the etching step (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of theoxide semiconductor film can be 5×10¹⁸ atoms/cm³ or lower (preferably1×10¹⁸ atoms/cm³ or lower).

Thus, the semiconductor devices including the transistors 4010 and 4011using an oxide semiconductor film and having stable electricalcharacteristics, illustrated in FIGS. 4A to 4C and FIGS. 6A and 6Baccording to this embodiment, can have high reliability. Further, such ahighly reliable semiconductor device can be manufactured with a highyield, so that high productivity can be achieved.

A conductive layer may be further provided so as to overlap with thechannel formation region of the oxide semiconductor film of thetransistor 4011 for the driver circuit. By providing the conductivelayer so as to overlap with the channel formation region of the oxidesemiconductor film, the amount of change in the threshold voltage of thetransistor 4011 from before to after a bias-temperature stress test (BTtest) can be further reduced. The conductive layer may have the samepotential as or a potential different from that of the gate electrodelayer of the transistor 4011, and can function as a second gateelectrode layer. The potential of the conductive layer may be GND or 0V, or the conductive layer may be in a floating state.

In addition, the conductive layer has a function of blocking an externalelectric field, that is, a function of preventing an external electricfield (particularly, a function of preventing static electricity) fromaffecting the inside (a circuit portion including a transistor). Theblocking function of the conductive layer can prevent fluctuation inelectrical characteristics of the transistor due to the influence of anexternal electric field such as static electricity.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. There is noparticular limitation on the kind of display element as long as displaycan be performed, and a variety of kinds of display elements can beemployed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 6A. In FIG. 6A, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is sandwichedtherebetween. The second electrode layer 4031 is provided on the secondsubstrate 4006 side, and the first electrode layer 4030 and the secondelectrode layer 4031 are stacked with the liquid crystal layer 4008provided therebetween.

A spacer 4035 is a columnar spacer which is obtained by selectiveetching of an insulating film, and is provided in order to control thethickness of the liquid crystal layer 4008 (cell gap). Alternatively, aspherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material (liquid crystalcomposition) exhibits a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions.

Alternatively, a liquid crystal composition exhibiting a blue phase forwhich an alignment film is unnecessary may be used for the liquidcrystal layer 4008. In this case, the liquid crystal layer 4008 is incontact with the first electrode layer 4030 and the second electrodelayer 4031. A blue phase is one of liquid crystal phases, which isgenerated just before a cholesteric phase changes into an isotropicphase while the temperature of a cholesteric liquid crystal isincreased. The blue phase can be exhibited using a liquid crystalcomposition which is a mixture of a liquid crystal and a chiral agent.To increase the temperature range where the blue phase is exhibited, aliquid crystal layer may be formed by adding a polymerizable monomer, apolymerization initiator, and the like to a liquid crystal compositionexhibiting a blue phase and by performing polymer stabilizationtreatment. The liquid crystal composition exhibiting a blue phase has ashort response time, and has optical isotropy, which makes the alignmentprocess unnecessary and the viewing angle dependence small. In addition,since an alignment film does not need to be provided and rubbingtreatment is unnecessary, electrostatic discharge damage caused by therubbing treatment can be prevented and defects and damage of the liquidcrystal display device in the manufacturing process can be reduced.Thus, productivity of the liquid crystal display device can be improved.A transistor including an oxide semiconductor film has a possibilitythat the electrical characteristics of the transistor may fluctuatesignificantly by the influence of static electricity and deviate fromthe designed range. Thus, it is more effective to use a liquid crystalcomposition exhibiting a blue phase for the liquid crystal displaydevice including the transistor including an oxide semiconductor film.

The specific resistivity of the liquid crystal material is higher thanor equal to 1×10⁹ Ω·cm, preferably higher than or equal to 1×10¹¹ Ω·cm,further preferably higher than or equal to 1×10¹² Ω·cm. Note that thespecific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of the transistor or the like. Byusing the transistor including an oxide semiconductor film, which isdisclosed in this specification, it is enough to provide a storagecapacitor having a capacitance that is ⅓ or less, preferably ⅕ or lessof liquid crystal capacitance of each pixel.

In the transistor including an oxide semiconductor film, which isdisclosed in this specification, the current in an off state (off-statecurrent) can be made small. Accordingly, an electric signal such as animage signal can be held for a longer period, and a writing interval canbe set longer in an on state. Thus, the frequency of refresh operationcan be reduced, which leads to an effect of suppressing powerconsumption.

The transistor including an oxide semiconductor film, which is disclosedin this specification, can have relatively high field-effect mobilityand thus can operate at high speed. For example, when such a transistorwhich can operate at high speed is used for a liquid crystal displaydevice, a switching transistor in a pixel portion and a drivertransistor in a driver circuit portion can be formed over one substrate.That is, since a semiconductor device formed of a silicon wafer or thelike is not additionally needed as a driver circuit, the number ofcomponents of the semiconductor device can be reduced. In addition, byusing the transistor which can operate at high speed in the pixelportion, a high-quality image can be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, ananti-ferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modemay be used. Some examples can be given as the vertical alignment mode,and for example a multi-domain vertical alignment (MVA) mode, apatterned vertical alignment (PVA) mode, or an advanced super view (ASV)mode can be used. This embodiment can also be applied to a VA liquidcrystal display device. The VA mode is a kind of mode in which alignmentof liquid crystal molecules of a liquid crystal display panel iscontrolled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied to the display device. It is alsopossible to use a method called domain multiplication or multi-domaindesign, in which a pixel is divided into some regions (subpixels) andmolecules are aligned in different directions in the respective regions.

In the display device, a black matrix (light-blocking layer), an opticalmember (optical substrate) such as a polarizing member, a retardationmember, or an anti-reflection member, and the like are provided asappropriate. For example, circular polarization may be obtained by usinga polarizing substrate and a retardation substrate. In addition, abacklight, a side light, or the like may be used as a light source.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors of R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. The sizes of display regions may differ betweenrespective dots of color elements. Note that one embodiment of theinvention disclosed herein is not limited to the application to adisplay device for color display and can also be applied to a displaydevice for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element. In this embodiment, an example of using anorganic EL element as a light-emitting element is described.

Inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

To extract light emitted from the light-emitting element, at least oneof the pair of electrodes has a light-transmitting property. Atransistor and the light-emitting element are formed over a substrate.The light-emitting element can have any of the following structures: atop emission structure in which light emission is extracted through asurface opposite to the substrate; a bottom emission structure in whichlight emission is extracted through a surface on the substrate side; anda dual emission structure in which light emission is extracted throughthe surface opposite to the substrate and the surface on the substrateside.

FIGS. 5A and 5B and FIG. 6B illustrate examples of a light-emittingdevice including a light-emitting element as a display element.

FIG. 5A is a plan view of a light-emitting device and FIG. 5B is across-sectional view taken along dashed-dotted lines V1-W1, V2-W2, andV3-W3 in FIG. 5A. Note that, an electroluminescent layer 542 and asecond electrode layer 543 are not illustrated in the plan view in FIG.5A.

The light-emitting device illustrated in FIGS. 5A and 5B includes, overa substrate 500, a transistor 510, a capacitor 520, and an intersection530 of wiring layers. The transistor 510 is electrically connected to alight-emitting element 540. Note that FIGS. 5A and 5B illustrate abottom-emission light-emitting device in which light from thelight-emitting element 540 is extracted through the substrate 500.

The transistor described in any of Embodiments 1 to 3 can be applied tothe transistor 510. In this embodiment, an example in which a transistorwhose structure and manufacturing method are similar to those of thetransistor 440 described in Embodiment 1 is used will be described.

The transistor 510 includes gate electrode layers 511 a and 511 b, agate insulating film 502, an oxide semiconductor film 512, andconductive layers 513 a and 513 b functioning as a source electrodelayer and a drain electrode layer.

In manufacturing the transistor 510 whose structure and manufacturingmethod are similar to those of the transistor 440 described inEmbodiment 1, after the conductive layers 513 a and 513 b functioning asthe source electrode layer and the drain electrode layer are formed, astep of removing the residue existing on the surface of the oxidesemiconductor film 512 and in the vicinity of the surface between theconductive layer 513 a and the conductive layer 513 b functioning as thesource electrode layer and the drain electrode layer is performed.Treatment using water or an alkaline solution or plasma treatment can beperformed as the residue removal step. For example, treatment usingwater or a TMAH solution, plasma treatment using oxygen, dinitrogenmonoxide, or a rare gas (typically argon), or the like can be favorablyused. Alternatively, treatment using dilute hydrofluoric acid may beused.

As described in Embodiment 2, after the formation of the gate electrodelayers 511 a and 511 b, the step of removing the residue caused on thesurfaces of the gate electrode layers 511 a and 511 b and in thevicinity of the surfaces due to the etching step may be performed.Further, as described in Embodiment 3, the step of removing the residueon the surfaces of the gate electrode layers 511 a and 511 b and in thevicinity of the surfaces may be performed after the formation of thegate electrode layers 511 a and 511 b, and then the step of removing theresidue on the surface of the oxide semiconductor film 512 and in thevicinity of the surface may be performed after the formation of theconductive layers 513 a and 513 b.

Since the surface of the oxide semiconductor film 512 and the vicinityof the surface can be prevented from being contaminated by the residue,in the transistor 510, the surface density of the impurities due to theetching step (typically halogen (e.g., chlorine, fluorine), boron,phosphorus, aluminum, iron, or carbon) on the surface of the oxidesemiconductor film 512 can be 1×10¹³ atoms/cm² or lower (preferably1×10¹² atoms/cm² or lower). Further, the concentration of the impuritiesdue to the etching step (typically halogen (e.g., chlorine, fluorine),boron, phosphorus, aluminum, iron, or carbon) on the surface of theoxide semiconductor film 512 can be 5×10¹⁸ atoms/cm³ or lower(preferably 1×10¹⁸ atoms/cm³ or lower).

Thus, the semiconductor device including the transistor 510 using theoxide semiconductor film 512 and having stable electricalcharacteristics, illustrated in FIGS. 5A and 5B according to thisembodiment, can have high reliability. Further, such a highly reliablesemiconductor device can be manufactured with a high yield, so that highproductivity can be achieved.

The capacitor 520 includes conductive layers 521 a and 521 b, the gateinsulating film 502, an oxide semiconductor film 522, and a conductivelayer 523. The gate insulating film 502 and the oxide semiconductor film522 are sandwiched between the conductive layer 523 and the conductivelayers 521 a and 521 b, so that the capacitor is formed.

The intersection 530 of wiring layers is an intersection of a conductivelayer 533 and the gate electrode layers 511 a and 511 b. The conductivelayer 533 and the gate electrode layers 511 a and 511 b intersect witheach other with the gate insulating film 502 provided therebetween.

In this embodiment, a 30-nm-thick titanium film is used as the gateelectrode layer 511 a and the conductive layer 521 a, and a 200-nm-thickcopper thin film is used as the gate electrode layer 511 b and theconductive layer 521 b. Thus, the gate electrode layer has astacked-layer structure of a titanium film and a copper thin film.

A 25-nm-thick IGZO film is used as the oxide semiconductor films 512 and522.

An interlayer insulating film 504 is formed over the transistor 510, thecapacitor 520, and the intersection 530 of wiring layers. Over theinterlayer insulating film 504, a color filter layer 505 is provided ina region overlapping with the light-emitting element 540. An insulatingfilm 506 functioning as a planarization insulating film is provided overthe interlayer insulating film 504 and the color filter layer 505.

The light-emitting element 540 having a stacked-layer structure in whicha first electrode layer 541, the electroluminescent layer 542, and thesecond electrode layer 543 are stacked in that order is provided overthe insulating film 506. The first electrode layer 541 and theconductive layer 513 a are in contact with each other in an openingformed in the insulating film 506 and the interlayer insulating film504, which reaches the conductive layer 513 a; thus the light-emittingelement 540 and the transistor 510 are electrically connected to eachother. Note that a partition 507 is provided so as to cover part of thefirst electrode layer 541 and the opening.

As the interlayer insulating film 504, a silicon oxynitride film havinga thickness greater than or equal to 200 nm and less than or equal to600 nm, which is formed by a plasma CVD method can be used. Further, aphotosensitive acrylic film having a thickness of 1500 nm and aphotosensitive polyimide film having a thickness of 1500 nm can be usedas the insulating film 506 and the partition 507, respectively.

As the color filter layer 505, for example, a chromaticlight-transmitting resin can be used. As such a chromaticlight-transmitting resin, a photosensitive organic resin or anonphotosensitive organic resin can be used. A photosensitive organicresin layer is preferably used, because the number of resist masks canbe reduced, leading to simplification of a process.

Chromatic colors are colors except achromatic colors such as black,gray, and white. The color filter layer is formed using a material whichtransmits only light of the chromatic color. As chromatic color, red,green, blue, or the like can be used. Cyan, magenta, yellow, or the likemay also be used. “Transmitting only light of the chromatic color” meansthat the light transmitted through the color filter layer has a peak ata wavelength of light of the chromatic color. The thickness of the colorfilter layer may be controlled to be optimal as appropriate inconsideration of the relationship between the concentration of acoloring material to be contained and the transmittance of light. Forexample, the color filter layer 505 may have a thickness greater than orequal to 1500 nm and less than or equal to 2000 nm.

In the light-emitting device illustrated in FIG. 6B, a light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the shown stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031. Thestructure of the light-emitting element 4513 can be changed asappropriate depending on a direction in which light is extracted fromthe light-emitting element 4513, or the like.

Partitions 4510 and 507 can be formed using an organic insulatingmaterial or an inorganic insulating material. It is particularlypreferable that the partitions 4510 and 507 be formed using aphotosensitive resin material to have openings over the first electrodelayers 4030 and 541, respectively. A sidewall of each opening is formedas a tilted surface with continuous curvature.

The electroluminescent layers 4511 and 542 may be formed using either asingle layer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition 4510 and over the second electrode layer 543 and thepartition 507 in order to prevent entry of oxygen, hydrogen, moisture,carbon dioxide, or the like into the light-emitting elements 4513 and540. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

Further, the light-emitting elements 4513 and 540 may be covered withrespective layers containing an organic compound deposited by anevaporation method so that oxygen, hydrogen, moisture, carbon dioxide,or the like do not enter the light-emitting elements 4513 and 540.

In addition, in a space which is sealed with the first substrate 4001,the second substrate 4006, and the sealant 4005, a filler 4514 isprovided. In this manner, it is preferable that a panel be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. For example, nitrogen is used as the filler.

In addition, as needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the first particles and the second particles in the microcapsules movein opposite directions to each other and only the color of the particlesgathering on one side is displayed. Note that the first particles andthe second particles each contain pigment and do not move without anelectric field. Moreover, the first particles and the second particleshave different colors (which may be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

In FIGS. 4A to 4C, FIGS. 5A and 5B, and FIGS. 6A and 6B, a flexiblesubstrate as well as a glass substrate can be used as any of the firstsubstrates 4001 and 500 and the second substrate 4006. For example, aplastic substrate having a light-transmitting property or the like canbe used. As the plastic substrate, a fiberglass-reinforced plastics(FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or anacrylic resin film can be used. In the case where a light-transmittingproperty is not needed, a metal substrate (metal film) of aluminum,stainless steel, or the like may be used. For example, a sheet with astructure in which an aluminum foil is sandwiched between PVF films orpolyester films can be used.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 4020. The insulating film 4020 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 4020 over theoxide semiconductor film has a high shielding effect (blocking effect)of preventing penetration of both oxygen and impurities such as hydrogenand moisture.

Thus, in and after the manufacturing process, the aluminum oxide filmfunctions as a protective film for preventing entry of impurities suchas hydrogen and moisture, which can cause a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

The insulating films 4021 and 506 serving as planarization insulatingfilms can be formed using an organic material having heat resistance,such as an acrylic-, polyimide-, or benzocyclobutene-based resin,polyamide, or epoxy. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. Note that the insulating film may be formedby stacking a plurality of insulating films formed using any of thesematerials.

There is no particular limitation on the methods of forming theinsulating films 4021 and 506, and the following method or tool(equipment) can be used depending on the material: a sputtering method,an SOG method, spin coating, dipping, spray coating, a droplet dischargemethod (such as an inkjet method), a printing method (such as screenprinting or offset printing), a doctor knife, a roll coater, a curtaincoater, a knife coater, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Thus, the substrate and the thin films suchas the insulating film and the conductive film provided for the pixelportion where light is transmitted all have light-transmittingproperties with respect to light in the visible light wavelength range.

The first electrode layer and the second electrode layer (which may becalled a pixel electrode layer, a common electrode layer, a counterelectrode layer, or the like) for applying voltage to the displayelement may have light-transmitting properties or light-reflectingproperties, which depends on the direction in which light is extracted,the position where the electrode layer is provided, the patternstructure of the electrode layer, and the like.

The first electrode layers 4030 and 541 and the second electrode layers4031 and 543 can be formed using a light-transmitting conductivematerial such as indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, indium tin oxide towhich silicon oxide is added, or graphene.

The first electrode layers 4030 and 541 and the second electrode layers4031 and 543 can be formed using one or plural kinds selected frommetals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium(Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt(Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper(Cu), and silver (Ag); alloys thereof and nitrides thereof.

In this embodiment, since the light-emitting device illustrated in FIGS.5A and 5B has a bottom-emission structure, the first electrode layer 541has a light-transmitting property and the second electrode layer 543 hasa light-reflecting property. Accordingly, in the case of using a metalfilm as the first electrode layer 541, the film is preferably thinenough to secure a light-transmitting property; and in the case of usinga light-transmissive conductive film as the second electrode layer 543,a conductive film having a light-reflecting property is preferablystacked therewith.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayers 4030 and 541 and the second electrode layers 4031 and 543. As theconductive high molecule, what is called a π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more of aniline, pyrrole, andthiophene or a derivative thereof can be given.

Since the transistor is easily broken by static electricity or the like,a protection circuit for protecting the driver circuit is preferablyprovided. The protection circuit is preferably formed using a nonlinearelement.

As described above, by using the transistor described in any ofEmbodiments 1 to 3, a semiconductor device having a variety of functionscan be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 5

A semiconductor device having an image sensor function of readinginformation on an object can be manufactured using the transistordescribed in any of Embodiments 1 to 3.

FIG. 7A illustrates an example of a semiconductor device having an imagesensor function. FIG. 7A is an equivalent circuit diagram of aphotosensor, and FIG. 7B is a cross-sectional view of part of thephotosensor.

One electrode of a photodiode 602 is electrically connected to aphotodiode reset signal line 658, and the other electrode of thephotodiode 602 is electrically connected to a gate of a transistor 640.One of a source and a drain of the transistor 640 is electricallyconnected to a photosensor reference signal line 672, and the other ofthe source and the drain of the transistor 640 is electrically connectedto one of a source and a drain of a transistor 656. A gate of thetransistor 656 is electrically connected to a gate signal line 659, andthe other of the source and the drain thereof is electrically connectedto a photosensor output signal line 671.

Note that in a circuit diagram in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. In FIG. 7A, the transistor 640 and the transistor656 are each a transistor including an oxide semiconductor film, towhich the transistor described in any of Embodiments 1 to 3 can beapplied. Described in this embodiment is an example in which atransistor whose structure and manufacturing method are similar to thoseof the transistor 440 described in Embodiment 1 is applied.

FIG. 7B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photosensor. The transistor 640 and the photodiode602 functioning as a sensor are provided over a substrate 601 (TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with the use of anadhesive layer 608.

An insulating film 631, an interlayer insulating film 633, and aninterlayer insulating film 634 are provided over the transistor 640. Thephotodiode 602 is provided over the interlayer insulating film 633. Inthe photodiode 602, a first semiconductor film 606 a, a secondsemiconductor film 606 b, and a third semiconductor film 606 c aresequentially stacked from the interlayer insulating film 633 side,between an electrode layer 642 formed over the interlayer insulatingfilm 634 and electrode layers 641 a and 641 b formed over the interlayerinsulating film 633.

The electrode layer 641 b is electrically connected to a conductivelayer 643 formed over the interlayer insulating film 634, and theelectrode layer 642 is electrically connected to a conductive layer 645through the electrode layer 641 a. The conductive layer 645 iselectrically connected to the gate electrode layer of the transistor640, and the photodiode 602 is electrically connected to the transistor640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity as the first semiconductor film 606 a, a high-resistancesemiconductor film (i-type semiconductor film) as the secondsemiconductor film 606 b, and a semiconductor film having n-typeconductivity as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor film 606a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (e.g.,boron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then an impurity elementmay be introduced into the amorphous silicon film by a diffusion methodor an ion implantation method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like to diffuse the impurity element. In this case, as a method forforming the amorphous silicon film, an LPCVD method, a vapor depositionmethod, a sputtering method, or the like may be used. The firstsemiconductor film 606 a is preferably formed to have a thicknessgreater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method with the use ofa semiconductor source gas. As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor film 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity. The third semiconductor film 606 c isformed by a plasma CVD method with the use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then an impurity elementmay be introduced into the amorphous silicon film by a diffusion methodor an ion implantation method. Heating or the like may be conductedafter introducing the impurity element by an ion implantation method orthe like to diffuse the impurity element. In this case, as a method forforming the amorphous silicon film, an LPCVD method, a vapor depositionmethod, a sputtering method, or the like may be used. The thirdsemiconductor film 606 c is preferably formed to have a thicknessgreater than or equal to 20 nm and less than or equal to 200 nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily formed usingan amorphous semiconductor, and may be formed using a polycrystallinesemiconductor or a microcrystalline semiconductor (semi-amorphoussemiconductor: SAS).

The mobility of holes generated by the photoelectric effect is lowerthan the mobility of electrons. Thus, a pin photodiode has bettercharacteristics when a surface on the p-type semiconductor film side isused as a light-receiving plane. Here, an example in which light thatpasses through a surface of the substrate 601, over which the pinphotodiode is formed, is received and converted into electric signals bythe photodiode 602 is described. Further, light from the semiconductorfilm having a conductivity type opposite to that of the semiconductorfilm on the light-receiving plane is disturbance light; thus, theelectrode layer is preferably formed using a light-blocking conductivefilm. Note that a surface on the n-type semiconductor film side canalternatively be used as the light-receiving plane.

With the use of an insulating material, the insulating film 631, theinterlayer insulating film 633 and the interlayer insulating film 634can be formed, depending on the material, by a sputtering method, aplasma CVD method, an SOG method, spin coating, dipping, spray coating,a droplet discharge method (such as an inkjet method), a printing method(such as screen printing or offset printing), or the like.

The insulating film 631 can be formed using an inorganic insulatingmaterial and can have a single-layer structure or a stacked-layerstructure including any of oxide insulating films such as a siliconoxide layer, a silicon oxynitride layer, an aluminum oxide layer, and analuminum oxynitride layer; and nitride insulating films such as asilicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, and an aluminum nitride oxide layer.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 631. The insulating film 631 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 631 over theoxide semiconductor film has a high shielding effect (blocking effect)of preventing penetration of both oxygen and impurities such as hydrogenand moisture.

Thus, in and after the manufacturing process, the aluminum oxide filmfunctions as a protective film for preventing entry of impurities suchas hydrogen and moisture, which can cause a change in characteristics,into the oxide semiconductor film and release of oxygen, which is a maincomponent material of the oxide semiconductor, from the oxidesemiconductor film.

To reduce surface roughness, an insulating film functioning as aplanarization insulating film is preferably used as each of theinterlayer insulating films 633 and 634. For the interlayer insulatingfilms 633 and 634, for example, an organic insulating material havingheat resistance, such as polyimide, acrylic resin, abenzocyclobutene-based resin, polyamide, or an epoxy resin, can be used.Other than such organic insulating materials, it is possible to use asingle layer or stacked layers of a low-dielectric constant material(low-k material), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

By detection of light 622 that enters the photodiode 602, information ona detection object can be read. Note that a light source such as abacklight can be used at the time of reading information on a detectedobject.

In manufacturing the transistor 640 whose structure and manufacturingmethod are similar to those of the transistor 440 described inEmbodiment 1, after the source electrode layer and the drain electrodelayer are formed, a step of removing the residue existing on the surfaceof the oxide semiconductor film and in the vicinity of the surfacebetween the source electrode layer and the drain electrode layer isperformed.

Treatment using water or an alkaline solution or plasma treatment can beperformed as the residue removal step. For example, treatment usingwater or a TMAH solution, plasma treatment using oxygen, dinitrogenmonoxide, or a rare gas (typically argon), or the like can be favorablyused. Alternatively, treatment using dilute hydrofluoric acid may beused.

As described in Embodiment 2, after the formation of the gate electrodelayer, the step of removing the residue caused on the surface of thegate electrode layer and in the vicinity of the surface due to theetching step may be performed. Further, as described in Embodiment 3,the step of removing the residue on the surface of the gate electrodelayer and in the vicinity of the surface may be performed after theformation of the gate electrode layer, and then the step of removing theresidue on the surface of the oxide semiconductor film and in thevicinity of the surface may be performed after the formation of thesource and drain electrode layers.

Since the surface of the oxide semiconductor film and the vicinity ofthe surface can be prevented from being contaminated by the residue, inthe transistors 640, the surface density of the impurities due to theetching step (typically halogen (e.g., chlorine, fluorine), boron,phosphorus, aluminum, iron, or carbon) on the surface of the oxidesemiconductor film can be 1×10¹³ atoms/cm² or lower (preferably 1×10¹²atoms/cm² or lower). Further, the concentration of the impurities due tothe etching step (typically halogen (e.g., chlorine, fluorine), boron,phosphorus, aluminum, iron, or carbon) on the surface of the oxidesemiconductor film can be 5×10¹⁸ atoms/cm³ or lower (preferably 1×10¹⁸atoms/cm³ or lower).

Thus, the semiconductor device including the transistor 640 using anoxide semiconductor film and having stable electrical characteristicsaccording to this embodiment can have high reliability. Further, such ahighly reliable semiconductor device can be manufactured with a highyield, so that high productivity can be achieved.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 6

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices include a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, cameras such as a digital camera and a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, an audio reproducing device, a game machine (e.g.,a pachinko machine or a slot machine), a game console, and the like.Specific examples of such electronic devices are illustrated in FIGS. 8Ato 8C.

FIG. 8A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, thehousing 9001 is provided with a power cord 9005 for supplying power.

The semiconductor device described in any of Embodiments 1 to 5 can beused in the display portion 9003 so that the electronic device can havehigh reliability.

The display portion 9003 has a touch-input function. When displayedbuttons 9004 which are displayed on the display portion 9003 of thetable 9000 are touched with his/her finger or the like, operation of thescreen and input of information can be carried out. Further, the table9000 may be made to communicate with home appliances or control the homeappliances, so that the table 9000 can function as a control devicewhich controls the home appliances by operation on the screen. Forexample, with use of the semiconductor device having an image sensingfunction described in Embodiment 5, the display portion 9003 can have atouch input function.

Further, it is possible to stand the screen of the display portion 9003so as to be perpendicular to a floor by using a hinge on the housing9001; thus, the table 9000 can also be used as a television set. When atelevision set having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 8B illustrates a television set 9100. In the television set 9100, adisplay portion 9103 is incorporated in a housing 9101 and an image canbe displayed on the display portion 9103. Note that the housing 9101 issupported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with operation keys 9109 of the remote controller 9110so that an image displayed on the display portion 9103 can becontrolled. Further, the remote controller 9110 may be provided with adisplay portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 8B is provided with areceiver, a modem, and the like. With the receiver, the television set9100 can receive a general television broadcast. Further, when thetelevision set 9100 is connected to a communication network with orwithout wires connection via the modem, one-way (from a transmitter to areceiver) or two-way (between a transmitter and a receiver or betweenreceivers) data communication can be performed.

The semiconductor device described in any of Embodiments 1 to 5 can beused in the display portions 9103 and 9107 so that the television setand the remote controller can have high reliability.

FIG. 8C illustrates a computer which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of Embodiments 1 to 5 can beused in the display portion 9203, in which case, the computer can havehigh reliability.

FIGS. 9A and 9B illustrate a tablet terminal that can be folded. In FIG.9A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-mode switch9034, a power switch 9035, a power-saving-mode switch 9036, a clip 9033,and an operation switch 9038.

The semiconductor device described in any of Embodiments 1 to 5 can beused in the display portion 9631 a and the display portion 9631 b sothat the tablet terminal can have high reliability.

A touch panel area 9632 a can be provided in part of the display portion9631 a, in which area, data can be input by touching displayed operationkeys 9638. In FIG. 9A, a half of the display portion 9631 a has only adisplay function and the other half has a touch panel function. However,one embodiment of the present invention is not limited to thisstructure, and the whole display portion 9631 a may have a touch panelfunction. For example, the display portion 9631 a can display a keyboardin the whole region to be used as a touch panel, and the display portion9631 b can be used as a display screen.

A touch panel area 9632 b can be provided in part of the display portion9631 b like in the display portion 9631 a. By touching a keyboarddisplay switching button 9639 displayed on the touch panel with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

Touch input can be performed concurrently on the touch panel area 9632 aand the touch panel area 9632 b.

The display-mode switch 9034 allows switching between a landscape modeand a portrait mode, color display and black-and-white display, and thelike. The power-saving-mode switch 9036 allows optimizing the displayluminance in accordance with the amount of external light in use whichis detected by an optical sensor incorporated in the tablet terminal. Inaddition to the optical sensor, another detecting device such as asensor for detecting inclination, like a gyroscope or an accelerationsensor, may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 9A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, higher definition images may be displayed on oneof the display portions 9631 a and 9631 b.

FIG. 9B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. Note that FIG.9B shows an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, which makes it possible to provide a tablet terminal withhigh durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 9A and 9B can have otherfunctions such as a function of displaying a variety of kinds of data(e.g., a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by a variety of kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630, sothat the battery 9635 can be charged efficiently. The use of a lithiumion battery as the battery 9635 is advantageous in downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 9B are described with reference to a blockdiagram of FIG. 9C. FIG. 9C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 9B.

First, description is given of an example of the operation in the casewhere power is generated by the solar battery 9633 using external light.The voltage of power generated by the solar battery is raised or loweredby the DCDC converter 9636 so that a voltage for charging the battery9635 is obtained. When the power from the solar battery 9633 is used forthe operation of the display portion 9631, the switch SW1 is turned onand the voltage of the power is raised or lowered by the converter 9637to a voltage needed for operating the display portion 9631. When displayis not performed on the display portion 9631, the switch SW1 is turnedoff and the switch SW2 is turned on so that the battery 9635 can becharged.

Although the solar battery 9633 is shown as an example of a chargemeans, there is no particular limitation on the charge means and thebattery 9635 may be charged with another means such as a piezoelectricelement or a thermoelectric conversion element (Peltier element). Forexample, the battery 9635 may be charged with a non-contact powertransmission module that transmits and receives power wirelessly(without contact) to charge the battery or with a combination of othercharging means.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Example 1

This example shows results of a residue removal step performed on asurface of a metal film and a surface of an oxide semiconductor film.

To make samples, an IGZO film with a thickness of 50 nm was formed as anoxide semiconductor film over a silicon substrate by a sputtering methodusing an oxide target with the following atomic ratio, In:Ga:Zn=3:1:2.Deposition conditions were as follows: an atmosphere of argon and oxygen(argon:oxygen=30 sccm: 15 sccm) was used, the pressure was 0.4 Pa, thepower supply was 0.5 kW, and the substrate temperature was 200° C.

Next, the oxide semiconductor film was subjected to etching treatment bya dry etching method (under such etching conditions that an etching gas(BCl₃:Cl₂=60 sccm: 20 sccm) was used, the ICP power supply was 450 W,the bias power was 100 W, and the pressure was 1.9 Pa). Thus, a sampleA-1 was made.

After the etching treatment, water treatment was performed. Thus, asample A-2 was made.

The IGZO film on which the water treatment was performed was subjectedto plasma treatment using oxygen (under such conditions that a gas(O₂=300 sccm) was used, the power supply was 1800 W, the pressure was66.5 Pa, and the treatment time was 3 minutes). Thus, a sample B-1 wasmade. The IGZO film on which the water treatment was performed wassubjected to plasma treatment using dinitrogen monoxide (under suchconditions that a gas (N₂O=200 sccm) was used, the power supply was 100W, the power source frequency was 27 MHz, the pressure was 40 Pa, thesubstrate temperature was 350° C., and the treatment time was 25minutes). Thus, a sample B-2 was made. The IGZO film on which the watertreatment was performed was subjected to treatment using a TMAH solution(under such conditions that the temperature was 50° C. and the treatmenttime was 60 seconds). Thus, a sample B-3 was made. The IGZO film onwhich the water treatment was performed was subjected to treatment usingan ammonia hydrogen peroxide (H₂O:ammonia:hydrogen peroxide=2:2:5)(under such conditions that the temperature was room temperature and thetreatment time was 10 seconds). Thus, a sample B-4 was made. The IGZOfilm on which the water treatment was performed was subjected to plasmatreatment using oxygen (under such conditions that a gas (O₂=200 sccm)was used, the power supply was 100 W, the power source frequency was 27MHz, the pressure was 40 Pa, and the substrate temperature was 350° C.,and the treatment time was 2 minutes). Thus, a sample B-5 was made. Notethat the sample B-1 and the sample B-5 are samples subjected to plasmatreatment using oxygen under different treatment conditions.

The surface density of chlorine on film surfaces of the samples A-1,A-2, B-1 to B-5 measured by total reflection X-ray fluorescencespectroscopy is shown in Table 1 and Table 2.

TABLE 1 Surface Density of Name Chlorine (atoms/cm²) of MeasurementAfter After Sample Film Treatment Deposition Treatment A-1 IGZO Dryetching treatment 5.3E+11 8.1E+13 A-2 IGZO Dry etching treat- 8.8E+115.9E+12 ment + water treatment

TABLE 2 Surface Density of Chlorine (atoms/cm²) After Dry Etching AfterName Residue Treatment + Residue of Measurement Removal After WaterRemoval Sample Film Step Deposition Treatment Step B-1 IGZO O₂ plasma8.8E+11 5.9E+12 1.3E+12 treatment B-2 IGZO N₂O plasma 5.4E+11 5.2E+129.1E+11 treatment B-3 IGZO TMAH 2.1E+11 4.9E+12 2.5E+11 solutiontreatment B-4 IGZO Ammonia 2.8E+11 4.6E+12 4.4E+11 hydrogen peroxidetreatment B-5 IGZO O₂ plasma 4.7E+11 3.7E+12 1.2E+12 treatment

As for the sample A-1 which was not subjected to the residue removalstep after the dry etching, the surface density of chlorine on thesurface of the IGZO film was largely increased. In contrast, as for thesample A-2 which was subjected to the water treatment after the dryetching, the increase in the surface density of chlorine on the surfaceof the IGZO film was reduced.

As for the samples B-1 to B-5 which were subjected to the residueremoval step such as the plasma treatment using dinitrogen monoxide, thetreatment using a TMAH solution, the treatment using an ammonia hydrogenperoxide, or the plasma treatment using oxygen, the surface density ofchlorine on the surface of the IGZO film after the residue removal stepwas equal to or lower than 1×10¹³ atoms/cm². This shows that the residueremoval step further removed chlorine and prevented the increase of thesurface density of chlorine.

Next, to make different samples, a tungsten (W) film with a thickness of200 nm was formed over a glass substrate by a sputtering method as ametal film (under such deposition conditions that an argon (80 sccm)atmosphere was used, the pressure was 0.8 Pa, the power supply was 1 kW,and the substrate temperature was 230° C.).

Then, the tungsten film was subjected to etching treatment by a dryetching method (under such etching conditions that an etching gas(CF₄:Cl₂:O₂=25 sccm: 25 sccm: 10 sccm) was used; the ICP power supplywas 500 W; the bias power was 100 W; and the pressure was 1.0 Pa) so asto be etched by a thickness of about 50 nm.

After the etching treatment, water treatment was performed.

The tungsten film on which the water treatment was performed wassubjected to plasma treatment using oxygen (under such conditions that agas (O₂=300 sccm) was used, the power supply was 1800 W, the pressurewas 66.5 Pa, and the treatment time was 3 minutes). Thus, a sample C-1was made. The tungsten film on which the water treatment was performedwas subjected to plasma treatment using dinitrogen monoxide (under suchconditions that a gas (N₂O=200 sccm) was used, the power supply was 100W, the power source frequency was 27 MHz, the pressure was 40 Pa, thesubstrate temperature was 350° C., and the treatment time was 25minutes). Thus, a sample C-2 was made. The tungsten film on which thewater treatment was performed was subjected to treatment using a TMAHsolution (under such conditions that the temperature was 50° C. and thetreatment time was 60 seconds). Thus, a sample C-3 was made. Thetungsten film on which the water treatment was performed was subjectedto treatment using an ammonia hydrogen peroxide (H₂O:ammonia:hydrogenperoxide=2:2:5) (under such conditions that the temperature was roomtemperature and the treatment time was 10 seconds). Thus, a sample C-4was made. The tungsten film on which the water treatment was performedwas subjected to plasma treatment using oxygen (under such conditionsthat a gas (O₂=200 sccm) was used, the power supply was 100 W, the powersource frequency was 27 MHz, the pressure was 40 Pa, and the substratetemperature was 350° C., and the treatment time was 2 minutes). Thus, asample C-5 was made. Note that the sample C-1 and the sample C-5 aresamples subjected to plasma treatment using oxygen under differenttreatment conditions.

The surface density of chlorine on film surfaces of the samples C-1 toC-5 measured by total reflection X-ray fluorescence spectroscopy isshown in Table 3.

TABLE 3 Surface Density of Chlorine (atoms/cm²) After Dry Etching AfterName Residue Treatment + Residue of Measurement Removal After WaterRemoval Sample Film Step Deposition Treatment Step C-1 W O₂ plasma4.0E+11 5.2E+12 1.3E+12 treatment C-2 W N₂O plasma 6.8E+11 5.7E+126.5E+11 treatment C-3 W TMAH 5.2E+11 4.3E+12 2.2E+11 solution treatmentC-4 W Ammonia 6.8E+11 5.2E+12 2.3E+11 hydrogen peroxide treatment C-5 WO₂ plasma 6.9E+11 5.2E+12 1.0E+12 treatment

As for the samples C-1 to C-5 which were subjected to, after the dryetching and the water treatment, the residue removal step such as theplasma treatment using dinitrogen monoxide, the treatment using a TMAHsolution, the treatment using an ammonia hydrogen peroxide, or theplasma treatment using oxygen, the surface density of chlorine on thesurface of the tungsten film after the residue removal step was equal toor lower than 1×10¹³ atoms/cm². This shows that the residue removal stepremoved chlorine and prevented the increase of the surface density ofchlorine.

From the above results, the residue removal step such as the watertreatment, the plasma treatment using dinitrogen monoxide, the treatmentusing a TMAH solution, the treatment using an ammonia hydrogen peroxide,or the plasma treatment using oxygen has an effect of reducing theconcentration of impurities on the film surface which are caused due tothe etching step.

This application is based on Japanese Patent Application serial no.2011-230126 filed with the Japan Patent Office on Oct. 19, 2011, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a substrate; agate electrode layer over the substrate; an oxide semiconductor filmcomprising a channel formation region; and a gate insulating filmbetween the gate electrode layer and the oxide semiconductor film,wherein a concentration of chlorine on a top surface of the gateelectrode layer is 5×10¹⁸ atoms/cm³ or lower.
 2. The semiconductordevice according to claim 1, wherein the concentration of chlorine is1×10¹⁸ atoms/cm³ or lower.
 3. The semiconductor device according toclaim 1, wherein the oxide semiconductor film is located over the gateelectrode layer.
 4. The semiconductor device according to claim 1,wherein the oxide semiconductor film comprises indium, gallium and zinc.5. A semiconductor device comprising: a substrate; a gate electrodelayer over the substrate; an oxide semiconductor film comprising achannel formation region; and a gate insulating film between the gateelectrode layer and the oxide semiconductor film, wherein aconcentration of fluorine on a top surface of the gate electrode layeris 5×10¹⁸ atoms/cm³ or lower.
 6. The semiconductor device according toclaim 5, wherein the concentration of fluorine is 1×10¹⁸ atoms/cm³ orlower.
 7. The semiconductor device according to claim 5, wherein theoxide semiconductor film is located over the gate electrode layer. 8.The semiconductor device according to claim 5, wherein the oxidesemiconductor film comprises indium, gallium and zinc.
 9. Asemiconductor device comprising: a substrate; a gate electrode layerover the substrate; an oxide semiconductor film comprising a channelformation region; and a gate insulating film between the gate electrodelayer and the oxide semiconductor film, wherein a concentration ofchlorine on a top surface of the gate electrode layer is 5×10¹⁸atoms/cm³ or lower, and wherein the gate electrode layer comprisescopper.
 10. The semiconductor device according to claim 9, wherein thesubstrate is a glass substrate.
 11. The semiconductor device accordingto claim 9, wherein the substrate is a flexible substrate.
 12. Thesemiconductor device according to claim 9, wherein the oxidesemiconductor film comprises indium, gallium and zinc.
 13. Asemiconductor device comprising: a substrate; a gate electrode layerover the substrate; an oxide semiconductor film comprising a channelformation region; and a gate insulating film between the gate electrodelayer and the oxide semiconductor film, wherein a concentration ofchlorine on a top surface of the gate electrode layer is 1×10¹⁸atoms/cm³ or lower, wherein the gate electrode layer comprises copper,and wherein the oxide semiconductor film comprises indium, gallium andzinc.
 14. The semiconductor device according to claim 13, wherein thesubstrate is a glass substrate.
 15. The semiconductor device accordingto claim 13, wherein the substrate is a flexible substrate.